Re: [PATCH] KVM: x86: advertise absence of X86_BUG_NULL_SEG via CPUID

[Jim Mattson]

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
>