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KVM: x86: work around QEMU issue with synthetic CPUID leaves

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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: f144c49e8c ("KVM: x86: synthesize CPUID leaf 0x80000021h if useful")
Reported-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2022-04-29 14:43:04 -04:00
parent 643d95aac5
commit f751d8eac1
1 changed files with 14 additions and 5 deletions
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19
arch/x86/kvm/cpuid.c
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@ -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: