* [PATCH] KVM: x86: advertise absence of X86_BUG_NULL_SEG via CPUID
@ 2021-10-21 21:19 Paolo Bonzini
2021-10-21 21:36 ` Jim Mattson
0 siblings, 1 reply; 3+ messages in thread
From: Paolo Bonzini @ 2021-10-21 21:19 UTC (permalink / raw)
To: linux-kernel, kvm; +Cc: thomas.lendacky, dgilbert
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@redhat.com>
---
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.
+ */
+ 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);
+ 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
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] KVM: x86: advertise absence of X86_BUG_NULL_SEG via CPUID
2021-10-21 21:19 [PATCH] KVM: x86: advertise absence of X86_BUG_NULL_SEG via CPUID Paolo Bonzini
@ 2021-10-21 21:36 ` Jim Mattson
2021-10-22 8:27 ` Paolo Bonzini
0 siblings, 1 reply; 3+ messages in thread
From: Jim Mattson @ 2021-10-21 21:36 UTC (permalink / raw)
To: Paolo Bonzini; +Cc: linux-kernel, kvm, thomas.lendacky, dgilbert
On Thu, Oct 21, 2021 at 2:20 PM Paolo Bonzini <pbonzini@redhat.com> 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@redhat.com>
> ---
> 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
>
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] KVM: x86: advertise absence of X86_BUG_NULL_SEG via CPUID
2021-10-21 21:36 ` Jim Mattson
@ 2021-10-22 8:27 ` Paolo Bonzini
0 siblings, 0 replies; 3+ messages in thread
From: Paolo Bonzini @ 2021-10-22 8:27 UTC (permalink / raw)
To: Jim Mattson; +Cc: linux-kernel, kvm, thomas.lendacky, dgilbert
On 21/10/21 23:36, Jim Mattson wrote:
> On Thu, Oct 21, 2021 at 2:20 PM Paolo Bonzini <pbonzini@redhat.com> 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@redhat.com>
>> ---
>> 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.
Right I should use the name of the bit instead.
>> + 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?
Yes, that makes sense. Just wanted to gauge whether anybody thought it
a really bad idea.
Paolo
>
>> + 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
>>
>
^ permalink raw reply [flat|nested] 3+ messages in thread
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2021-10-21 21:19 [PATCH] KVM: x86: advertise absence of X86_BUG_NULL_SEG via CPUID Paolo Bonzini
2021-10-21 21:36 ` Jim Mattson
2021-10-22 8:27 ` Paolo Bonzini
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