On Thu, Oct 21, 2021 at 2:20 PM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: > > Guests have X86_BUG_NULL_SEG if and only if the host have it. Use > the info from static_cpu_has_bug to form the 0x80000021 CPUID leaf that > was defined for Zen3. Userspace can then set the bit even on older > CPUs that do not have the bug, such as Zen2. > > Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> > --- > arch/x86/kvm/cpuid.c | 17 ++++++++++++++++- > 1 file changed, 16 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c > index 2d70edb0f323..b51398e1727b 100644 > --- a/arch/x86/kvm/cpuid.c > +++ b/arch/x86/kvm/cpuid.c > @@ -902,7 +902,13 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function) > entry->edx = 0; > break; > case 0x80000000: > - entry->eax = min(entry->eax, 0x8000001f); > + entry->eax = min(entry->eax, 0x80000021); > + /* > + * X86_BUG_NULL_SEG is not reported in CPUID on Zen2; in > + * that case, provide the CPUID leaf ourselves. > + */ I think this is backwards. !X86_BUG_NULL_SEG isn't reported in CPUID on Zen2. > + if (!static_cpu_has_bug(X86_BUG_NULL_SEG)) > + entry->eax = max(entry->eax, 0x80000021); > break; > case 0x80000001: > cpuid_entry_override(entry, CPUID_8000_0001_EDX); > @@ -973,6 +979,15 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function) > entry->ebx &= ~GENMASK(11, 6); > } > break; > + case 0x80000020: > + entry->eax = entry->ebx = entry->ecx = entry->edx = 0; > + break; > + case 0x80000021: > + entry->ebx = entry->ecx = entry->edx = 0; > + entry->eax &= BIT(6); While we're here, shouldn't bit 0 (Processor ignores nested data breakpoints) and bit 2 (LFENCE is always dispatch serializing) also match the hardware? > + if (!static_cpu_has_bug(X86_BUG_NULL_SEG)) > + entry->eax |= BIT(6); > + break; > /*Add support for Centaur's CPUID instruction*/ > case 0xC0000000: > /*Just support up to 0xC0000004 now*/ > -- > 2.27.0 >
