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 <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
---
arch/x86/kvm/cpuid.c | 19 ++++++++++++++-----
1 file changed, 14 insertions(+), 5 deletions(-)
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:
--
2.31.1