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path: root/drivers/vhost/net.c
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2014-04-01vhost: don't open-code sockfd_put()Al Viro1-7/+7
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
2014-03-28vhost: validate vhost_get_vq_desc return valueMichael S. Tsirkin1-1/+5
vhost fails to validate negative error code from vhost_get_vq_desc causing a crash: we are using -EFAULT which is 0xfffffff2 as vector size, which exceeds the allocated size. The code in question was introduced in commit 8dd014adfea6f173c1ef6378f7e5e7924866c923 vhost-net: mergeable buffers support CVE-2014-0055 Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
2014-03-28vhost: fix total length when packets are too shortMichael S. Tsirkin1-0/+14
When mergeable buffers are disabled, and the incoming packet is too large for the rx buffer, get_rx_bufs returns success. This was intentional in order for make recvmsg truncate the packet and then handle_rx would detect err != sock_len and drop it. Unfortunately we pass the original sock_len to recvmsg - which means we use parts of iov not fully validated. Fix this up by detecting this overrun and doing packet drop immediately. CVE-2014-0077 Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
2014-02-13vhost: fix a theoretical race in device cleanupMichael S. Tsirkin1-0/+6
vhost_zerocopy_callback accesses VQ right after it drops a ubuf reference. In theory, this could race with device removal which waits on the ubuf kref, and crash on use after free. Do all accesses within rcu read side critical section, and synchronize on release. Since callbacks are always invoked from bh, synchronize_rcu_bh seems enough and will help release complete a bit faster. Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Acked-by: Jason Wang <jasowang@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
2014-02-13vhost: fix ref cnt checking deadlockMichael S. Tsirkin1-21/+20
vhost checked the counter within the refcnt before decrementing. It really wanted to know that it is the one that has the last reference, as a way to batch freeing resources a bit more efficiently. Note: we only let refcount go to 0 on device release. This works well but we now access the ref counter twice so there's a race: all users might see a high count and decide to defer freeing resources. In the end no one initiates freeing resources until the last reference is gone (which is on VM shotdown so might happen after a looooong time). Let's do what we probably should have done straight away: switch from kref to plain atomic, documenting the semantics, return the refcount value atomically after decrement, then use that to avoid the deadlock. Reported-by: Qin Chuanyu <qinchuanyu@huawei.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Acked-by: Jason Wang <jasowang@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
2013-12-06vhost: remove the dead branchZhi Yong Wu1-7/+2
Since vhost_dev_init() forever return 0, some branches are never run, therefore need to be removed. Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com> Acked-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
2013-09-03vhost_net: correctly limit the max pending buffersJason Wang1-11/+7
As Michael point out, We used to limit the max pending DMAs to get better cache utilization. But it was not done correctly since it was one done when there's no new buffers submitted from guest. Guest can easily exceeds the limitation by keeping sending packets. So this patch moves the check into main loop. Tests shows about 5%-10% improvement on per cpu throughput for guest tx. Signed-off-by: Jason Wang <jasowang@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
2013-09-03vhost_net: poll vhost queue after marking DMA is doneJason Wang1-4/+5
We used to poll vhost queue before making DMA is done, this is racy if vhost thread were waked up before marking DMA is done which can result the signal to be missed. Fix this by always polling the vhost thread before DMA is done. Signed-off-by: Jason Wang <jasowang@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
2013-09-03vhost_net: determine whether or not to use zerocopy at one timeJason Wang1-27/+20
Currently, even if the packet length is smaller than VHOST_GOODCOPY_LEN, if upend_idx != done_idx we still set zcopy_used to true and rollback this choice later. This could be avoided by determining zerocopy once by checking all conditions at one time before. Signed-off-by: Jason Wang <jasowang@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
2013-09-03vhost_net: use vhost_add_used_and_signal_n() in vhost_zerocopy_signal_used()Jason Wang1-5/+8
We tend to batch the used adding and signaling in vhost_zerocopy_callback() which may result more than 100 used buffers to be updated in vhost_zerocopy_signal_used() in some cases. So switch to use vhost_add_used_and_signal_n() to avoid multiple calls to vhost_add_used_and_signal(). Which means much less times of used index updating and memory barriers. 2% performance improvement were seen on netperf TCP_RR test. Signed-off-by: Jason Wang <jasowang@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
2013-09-03vhost_net: make vhost_zerocopy_signal_used() return voidJason Wang1-3/+2
None of its caller use its return value, so let it return void. Signed-off-by: Jason Wang <jasowang@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
2013-07-11vhost: Remove custom vhost rcu usageAsias He1-10/+6
Now, vq->private_data is always accessed under vq mutex. No need to play the vhost rcu trick. Signed-off-by: Asias He <asias@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
2013-07-11vhost-net: Always access vq->private_data under vq mutexAsias He1-11/+10
Signed-off-by: Asias He <asias@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
2013-07-07vhost: Make local function staticAsias He1-2/+2
$ make C=1 M=drivers/vhost drivers/vhost/net.c:168:5: warning: symbol 'vhost_net_set_ubuf_info' was not declared. Should it be static? drivers/vhost/net.c:194:6: warning: symbol 'vhost_net_vq_reset' was not declared. Should it be static? drivers/vhost/scsi.c:219:6: warning: symbol 'tcm_vhost_done_inflight' was not declared. Should it be static? Signed-off-by: Asias He <asias@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
2013-07-07vhost-net: fix use-after-free in vhost_net_flushMichael S. Tsirkin1-2/+7
vhost_net_ubuf_put_and_wait has a confusing name: it will actually also free it's argument. Thus since commit 1280c27f8e29acf4af2da914e80ec27c3dbd5c01 "vhost-net: flush outstanding DMAs on memory change" vhost_net_flush tries to use the argument after passing it to vhost_net_ubuf_put_and_wait, this results in use after free. To fix, don't free the argument in vhost_net_ubuf_put_and_wait, add an new API for callers that want to free ubufs. Acked-by: Asias He <asias@redhat.com> Acked-by: Jason Wang <jasowang@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
2013-06-11vhost: fix ubuf_info cleanupMichael S. Tsirkin1-15/+7
vhost_net_clear_ubuf_info didn't clear ubuf_info after kfree, this could trigger double free. Fix this and simplify this code to make it more robust: make sure ubuf info is always freed through vhost_net_clear_ubuf_info. Reported-by: Tommi Rantala <tt.rantala@gmail.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
2013-06-11vhost: check owner before we overwrite ubuf_infoMichael S. Tsirkin1-0/+4
If device has an owner, we shouldn't touch ubuf_info since it might be in use. Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
2013-06-10vhost_net: clear msg.control for non-zerocopy case during txJason Wang1-1/+2
When we decide not use zero-copy, msg.control should be set to NULL otherwise macvtap/tap may set zerocopy callbacks which may decrease the kref of ubufs wrongly. Bug were introduced by commit cedb9bdce099206290a2bdd02ce47a7b253b6a84 (vhost-net: skip head management if no outstanding). This solves the following warnings: WARNING: at include/linux/kref.h:47 handle_tx+0x477/0x4b0 [vhost_net]() Modules linked in: vhost_net macvtap macvlan tun nfsd exportfs bridge stp llc openvswitch kvm_amd kvm bnx2 megaraid_sas [last unloaded: tun] CPU: 5 PID: 8670 Comm: vhost-8668 Not tainted 3.10.0-rc2+ #1566 Hardware name: Dell Inc. PowerEdge R715/00XHKG, BIOS 1.5.2 04/19/2011 ffffffffa0198323 ffff88007c9ebd08 ffffffff81796b73 ffff88007c9ebd48 ffffffff8103d66b 000000007b773e20 ffff8800779f0000 ffff8800779f43f0 ffff8800779f8418 000000000000015c 0000000000000062 ffff88007c9ebd58 Call Trace: [<ffffffff81796b73>] dump_stack+0x19/0x1e [<ffffffff8103d66b>] warn_slowpath_common+0x6b/0xa0 [<ffffffff8103d6b5>] warn_slowpath_null+0x15/0x20 [<ffffffffa0197627>] handle_tx+0x477/0x4b0 [vhost_net] [<ffffffffa0197690>] handle_tx_kick+0x10/0x20 [vhost_net] [<ffffffffa019541e>] vhost_worker+0xfe/0x1a0 [vhost_net] [<ffffffffa0195320>] ? vhost_attach_cgroups_work+0x30/0x30 [vhost_net] [<ffffffffa0195320>] ? vhost_attach_cgroups_work+0x30/0x30 [vhost_net] [<ffffffff81061f46>] kthread+0xc6/0xd0 [<ffffffff81061e80>] ? kthread_freezable_should_stop+0x70/0x70 [<ffffffff817a1aec>] ret_from_fork+0x7c/0xb0 [<ffffffff81061e80>] ? kthread_freezable_should_stop+0x70/0x70 Signed-off-by: Jason Wang <jasowang@redhat.com> Acked-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
2013-05-06vhost-net: Cleanup vhost_ubuf and vhost_zcopyAsias He1-28/+30
- Rename vhost_ubuf to vhost_net_ubuf - Rename vhost_zcopy_mask to vhost_net_zcopy_mask - Make funcs static Signed-off-by: Asias He <asias@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
2013-05-06vhost: Move VHOST_NET_FEATURES to net.cAsias He1-0/+6
vhost.h should not depend on device specific marcos like VHOST_NET_F_VIRTIO_NET_HDR and VIRTIO_NET_F_MRG_RXBUF. Signed-off-by: Asias He <asias@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
2013-05-06vhost-net: Free ubuf when vhost_dev_set_owner failsAsias He1-6/+32
Signed-off-by: Asias He <asias@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
2013-05-01vhost: fix error handling in RESET_OWNER ioctlMichael S. Tsirkin1-1/+7
RESET_OWNER ioctl would leave the fd in a bad state if memory allocation failed: device is stopped but owner is not reset. Make state changes after allocating memory, such that a failed ioctl has no effect. Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
2013-05-01vhost: move per-vq net specific fields out to netMichael S. Tsirkin1-16/+27
This will remove the need for vhost scsi to pull in virtio-net.h. Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
2013-05-01vhost: move vhost-net zerocopy fields to net.cAsias He1-23/+141
On top of 'vhost: Allow device specific fields per vq', we can move device specific fields to device virt queue from vhost virt queue. Signed-off-by: Asias He <asias@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
2013-05-01vhost: Allow device specific fields per vqAsias He1-23/+41
This is useful for any device who wants device specific fields per vq. For example, tcm_vhost wants a per vq field to track requests which are in flight on the vq. Also, on top of this we can add patches to move things like ubufs from vhost.h out to net.c. Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Asias He <asias@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
2013-04-11vhost_net: remove tx polling stateJason Wang1-68/+6
After commit 2b8b328b61c799957a456a5a8dab8cc7dea68575 (vhost_net: handle polling errors when setting backend), we in fact track the polling state through poll->wqh, so there's no need to duplicate the work with an extra vhost_net_polling_state. So this patch removes this and make the code simpler. This patch also removes the all tx starting/stopping code in tx path according to Michael's suggestion. Netperf test shows almost the same result in stream test, but gets improvements on TCP_RR tests (both zerocopy or copy) especially on low load cases. Tested between multiqueue kvm guest and external host with two direct connected 82599s. zerocopy disabled: sessions|transaction rates|normalize| before/after/+improvements 1 | 9510.24/11727.29/+23.3% | 693.54/887.68/+28.0% | 25| 192931.50/241729.87/+25.3% | 2376.80/2771.70/+16.6% | 50| 277634.64/291905.76/+5% | 3118.36/3230.11/+3.6% | zerocopy enabled: sessions|transaction rates|normalize| before/after/+improvements 1 | 7318.33/11929.76/+63.0% | 521.86/843.30/+61.6% | 25| 167264.88/242422.15/+44.9% | 2181.60/2788.16/+27.8% | 50| 272181.02/294347.04/+8.1% | 3071.56/3257.85/+6.1% | Signed-off-by: Jason Wang <jasowang@redhat.com> Acked-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
2013-03-17vhost/net: fix heads usage of ubuf_infoMichael S. Tsirkin1-1/+2
ubuf info allocator uses guest controlled head as an index, so a malicious guest could put the same head entry in the ring twice, and we will get two callbacks on the same value. To fix use upend_idx which is guaranteed to be unique. Reported-by: Rusty Russell <rusty@rustcorp.com.au> Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Cc: stable@kernel.org Signed-off-by: David S. Miller <davem@davemloft.net>
2013-01-29vhost_net: handle polling errors when setting backendJason Wang1-9/+18
Currently, the polling errors were ignored, which can lead following issues: - vhost remove itself unconditionally from waitqueue when stopping the poll, this may crash the kernel since the previous attempt of starting may fail to add itself to the waitqueue - userspace may think the backend were successfully set even when the polling failed. Solve this by: - check poll->wqh before trying to remove from waitqueue - report polling errors in vhost_poll_start(), tx_poll_start(), the return value will be checked and returned when userspace want to set the backend After this fix, there still could be a polling failure after backend is set, it will addressed by the next patch. Signed-off-by: Jason Wang <jasowang@redhat.com> Acked-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
2013-01-29vhost_net: correct error handling in vhost_net_set_backend()Jason Wang1-5/+11
Currently, when vhost_init_used() fails the sock refcnt and ubufs were leaked. Correct this by calling vhost_init_used() before assign ubufs and restore the oldsock when it fails. Signed-off-by: Jason Wang <jasowang@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
2012-12-06vhost-net: enable zerocopy tx by defaultMichael S. Tsirkin1-2/+3
Zero copy TX has been around for a while now. We seem to be down to eliminating theoretical bugs and performance tuning at this point: it's probably time to enable it by default so that most users get the benefit. Keep the flag around meanwhile so users can experiment with disabling this if they experience regressions. I expect that we will remove it in the future. Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
2012-12-06vhost-net: skip head management if no outstandingMichael S. Tsirkin1-4/+7
For short packets zerocopy mode adds overhead of managing heads which isn't necessary: we could simly update used ring directly same as with zerocopy disabled. Things seem to run a bit faster if we detect and bypass head management when zcopy isn't used. Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
2012-12-06vhost-net: flush outstanding DMAs on memory changeMichael S. Tsirkin1-9/+19
When memory map changes, we need to flush outstanding DMAs as they might in theory reference old memory addresses. To do this simply stop initiating new DMAs and wait for ubufs ref count to drop to 0. Afterwards reset the count back to 1. Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
2012-12-06vhost: avoid backend flush on vring opsMichael S. Tsirkin1-2/+5
vring changes already do a flush internally where appropriate, so we do not need a second flush. It's currently not very expensive but a follow-up patch makes flush more heavy-weight, so remove the extra flush here to avoid regressing performance if call or kick fds are changed on data path. Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
2012-12-03vhost-net: initialize zcopy packet countersMichael S. Tsirkin1-0/+3
These packet counters are used to drive the zercopy selection heuristic so nothing too bad happens if they are off a bit - and they are also reset once in a while. But it's cleaner to clear them when backend is set so that we start in a known state. Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
2012-11-10Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/netDavid S. Miller1-1/+2
Conflicts: drivers/net/ethernet/broadcom/bnx2x/bnx2x_main.c Minor conflict between the BCM_CNIC define removal in net-next and a bug fix added to net. Based upon a conflict resolution patch posted by Stephen Rothwell. Signed-off-by: David S. Miller <davem@davemloft.net>
2012-11-02vhost-net: reduce vq polling on tx zerocopyMichael S. Tsirkin1-2/+12
It seems that to avoid deadlocks it is enough to poll vq before we are going to use the last buffer. This is faster than c70aa540c7a9f67add11ad3161096fb95233aa2e. Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
2012-11-02vhost-net: select tx zero copy dynamicallyMichael S. Tsirkin1-8/+53
Even when vhost-net is in zero-copy transmit mode, net core might still decide to copy the skb later which is somewhat slower than a copy in user context: data copy overhead is added to the cost of page pin/unpin. The result is that enabling tx zero copy option leads to higher CPU utilization for guest to guest and guest to host traffic. To fix this, suppress zero copy tx after a given number of packets triggered late data copy. Re-enable periodically to detect workload changes. Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
2012-11-02vhost: move -net specific code outMichael S. Tsirkin1-0/+45
Zerocopy handling code is vhost-net specific. Move it from vhost.c/vhost.h out to net.c Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
2012-11-02vhost-net: cleanup macros for DMA status trackingMichael S. Tsirkin1-1/+2
Better document macros for DMA tracking. Add an explicit one for DMA in progress instead of relying on user supplying len != 1. Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
2012-10-24vhost: fix mergeable bufs on BE hostsMichael S. Tsirkin1-1/+2
We copy head count to a 16 bit field, this works by chance on LE but on BE guest gets 0. Fix it up. Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Tested-by: Alexander Graf <agraf@suse.de> Cc: stable@vger.kernel.org Signed-off-by: David S. Miller <davem@davemloft.net>
2012-07-22vhost: Separate vhost-net features from vhost featuresStefan Hajnoczi1-2/+2
In order for other vhost devices to use the VHOST_FEATURES bits the vhost-net specific bits need to be moved to their own VHOST_NET_FEATURES constant. (Asias: Update drivers/vhost/test.c to use VHOST_NET_FEATURES) Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com> Cc: Zhi Yong Wu <wuzhy@cn.ibm.com> Cc: Michael S. Tsirkin <mst@redhat.com> Cc: Paolo Bonzini <pbonzini@redhat.com> Cc: Asias He <asias@redhat.com> Signed-off-by: Nicholas A. Bellinger <nab@risingtidesystems.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
2012-05-16Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/netDavid S. Miller1-1/+6
2012-05-11vhost-net: fix handle_rx buffer sizeBasil Gor1-1/+6
Take vlan header length into account, when vlan id is stored as vlan_tci. Otherwise tagged packets coming from macvtap will be truncated. Signed-off-by: Basil Gor <basil.gor@gmail.com> Acked-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
2012-05-02vhost_net: zerocopy: adding and signalling immediately when fully copiedJason Wang1-0/+2
When a packet were fully copied in zerocopy, we don't wait for the DMA done to mark the done flag, so after the packet were passed to lower device, we need to add used and signal guest immediately. Signed-off-by: Jason Wang <jasowang@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
2012-05-02vhost_net: re-poll only on EAGAIN or ENOBUFSJason Wang1-1/+2
Currently, we restart tx polling unconditionally when sendmsg() fails. This would cause unnecessary wakeups of vhost wokers and waste cpu utlization when evil userspace(guest driver) is able to hit EFAULT or EINVAL. The polling is only needed when the socket send buffer were exceeded or not enough memory. So fix this by restarting polling only when sendmsg() returns EAGAIN/ENOBUFS. Signed-off-by: Jason Wang <jasowang@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
2012-05-02vhost_net: zerocopy: fix possible NULL pointer dereference of vq->bufsJason Wang1-1/+1
When we want to disable vhost_net backend while there's a tx work, a possible NULL pointer defernece may happen we we try to deference the vq->bufs after vhost_net_set_backend() assign a NULL to it. As suggested by Michael, fix this by checking the vq->bufs instead of vhost_sock_zcopy(). Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
2012-04-13skbuff: struct ubuf_info callback type safetyMichael S. Tsirkin1-1/+1
The skb struct ubuf_info callback gets passed struct ubuf_info itself, not the arg value as the field name and the function signature seem to imply. Rename the arg field to ctx to match usage, add documentation and change the callback argument type to make usage clear and to have compiler check correctness. Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
2012-02-28vhost: fix release path lockdep checksMichael S. Tsirkin1-1/+1
We shouldn't hold any locks on release path. Pass a flag to vhost_dev_cleanup to use the lockdep info correctly. Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Tested-by: Sasha Levin <levinsasha928@gmail.com>
2012-01-13vhost-net: add module alias (v2.1)stephen hemminger1-3/+5
By adding some module aliases, programs (or users) won't have to explicitly call modprobe. Vhost-net will always be available if built into the kernel. It does require assigning a permanent minor number for depmod to work. Also: - use C99 style initialization. - add missing entry in documentation for loop-control Signed-off-by: Stephen Hemminger <shemminger@vyatta.com> Acked-By: Kay Sievers <kay.sievers@vrfy.org> Signed-off-by: David S. Miller <davem@davemloft.net>
2011-07-21vhost: handle wrap around in # of bufs mathShirley Ma1-3/+9
The meth for calculating the # of outstanding buffers gives incorrect results when vq->upend_idx wraps around zero. Fix that. Signed-off-by: Shirley Ma <xma@us.ibm.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
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