Re: [PATCH 3/3] x86/speculation: Support Automatic IBRS under virtualization

[Paolo Bonzini]

On 11/4/22 22:36, Kim Phillips wrote:
> @@ -730,6 +730,8 @@ void kvm_set_cpu_caps(void)
>   		0 /* SME */ | F(SEV) | 0 /* VM_PAGE_FLUSH */ | F(SEV_ES) |
>   		F(SME_COHERENT));
>   
> +	kvm_cpu_cap_mask(CPUID_8000_0021_EAX, F(AUTOIBRS));

This should also include bits 0, 2 and 6.  Feel free to add #defines for
them in cpuid.c if x86 maintainers do not want them in cpufeatures.h.

There should also be something like:

                if (static_cpu_has(X86_FEATURE_LFENCE_RDTSC))
                        kvm_cpu_cap_set(CPUID_8000_0021_EAX, F(AMD_LFENCE_RDTSC));
                if (!static_cpu_has_bug(X86_BUG_NULL_SEG))
                        kvm_cpu_cap_set(CPUID_8000_0021_EAX, F(NSCB);

so that...

> @@ -1211,12 +1213,13 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>  		 *    EAX      0      NNDBP, Processor ignores nested data breakpoints
>  		 *    EAX      2      LAS, LFENCE always serializing
>  		 *    EAX      6      NSCB, Null selector clear base
> +		 *    EAX      8      Automatic IBRS
>  		 *
>  		 * Other defined bits are for MSRs that KVM does not expose:
>  		 *   EAX      3      SPCL, SMM page configuration lock
>  		 *   EAX      13     PCMSR, Prefetch control MSR
>  		 */
> -		entry->eax &= BIT(0) | BIT(2) | BIT(6);
> +		entry->eax &= BIT(0) | BIT(2) | BIT(6) | BIT(8);
>  		if (static_cpu_has(X86_FEATURE_LFENCE_RDTSC))
>  			entry->eax |= BIT(2);
>  		if (!static_cpu_has_bug(X86_BUG_NULL_SEG))
>  			entry->eax |= BIT(6);

... these five lines become simply

	cpuid_entry_override(entry, CPUID_8000_0021_EAX);

In the end these should be two patches:

- kvm, x86: use CPU capabilities for CPUID[0x80000021].EAX
- kvm, x86: support AMD automatic IBRS

Thanks,

Paolo