From f751d8eac17692905cdd6935f72d523d8adf3b65 Mon Sep 17 00:00:00 2001 From: Paolo Bonzini Date: Fri, 29 Apr 2022 14:43:04 -0400 Subject: KVM: x86: work around QEMU issue with synthetic CPUID leaves Synthesizing AMD leaves up to 0x80000021 caused problems with QEMU, which assumes the *host* CPUID[0x80000000].EAX is higher or equal to what KVM_GET_SUPPORTED_CPUID reports. This causes QEMU to issue bogus host CPUIDs when preparing the input to KVM_SET_CPUID2. It can even get into an infinite loop, which is only terminated by an abort(): cpuid_data is full, no space for cpuid(eax:0x8000001d,ecx:0x3e) To work around this, only synthesize those leaves if 0x8000001d exists on the host. The synthetic 0x80000021 leaf is mostly useful on Zen2, which satisfies the condition. Fixes: f144c49e8c39 ("KVM: x86: synthesize CPUID leaf 0x80000021h if useful") Reported-by: Maxim Levitsky Signed-off-by: Paolo Bonzini --- arch/x86/kvm/cpuid.c | 19 ++++++++++++++----- 1 file changed, 14 insertions(+), 5 deletions(-) (limited to 'arch') diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c index b24ca7f4ed7c..598334ed5fbc 100644 --- a/arch/x86/kvm/cpuid.c +++ b/arch/x86/kvm/cpuid.c @@ -1085,12 +1085,21 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function) case 0x80000000: entry->eax = min(entry->eax, 0x80000021); /* - * Serializing LFENCE is reported in a multitude of ways, - * and NullSegClearsBase is not reported in CPUID on Zen2; - * help userspace by providing the CPUID leaf ourselves. + * Serializing LFENCE is reported in a multitude of ways, and + * NullSegClearsBase is not reported in CPUID on Zen2; help + * userspace by providing the CPUID leaf ourselves. + * + * However, only do it if the host has CPUID leaf 0x8000001d. + * QEMU thinks that it can query the host blindly for that + * CPUID leaf if KVM reports that it supports 0x8000001d or + * above. The processor merrily returns values from the + * highest Intel leaf which QEMU tries to use as the guest's + * 0x8000001d. Even worse, this can result in an infinite + * loop if said highest leaf has no subleaves indexed by ECX. */ - if (static_cpu_has(X86_FEATURE_LFENCE_RDTSC) - || !static_cpu_has_bug(X86_BUG_NULL_SEG)) + if (entry->eax >= 0x8000001d && + (static_cpu_has(X86_FEATURE_LFENCE_RDTSC) + || !static_cpu_has_bug(X86_BUG_NULL_SEG))) entry->eax = max(entry->eax, 0x80000021); break; case 0x80000001: -- cgit v1.2.3