X86_FEATURE_AMD_IBRS getting set for Intel guests

X86_FEATURE_AMD_IBRS getting set for Intel guests

[John Levon]

```
arch/x86/kernel/cpu/common.c:

 863         /*                                                                       
 864          * The Intel SPEC_CTRL CPUID bit implies IBRS and IBPB support,          
 865          * and they also have a different bit for STIBP support. Also,           
 866          * a hypervisor might have set the individual AMD bits even on           
 867          * Intel CPUs, for finer-grained selection of what's available.          
 868          */                                                                      
 869         if (cpu_has(c, X86_FEATURE_SPEC_CTRL)) {                                 
 870                 set_cpu_cap(c, X86_FEATURE_IBRS);                                
 871                 set_cpu_cap(c, X86_FEATURE_IBPB);                                
 872                 set_cpu_cap(c, X86_FEATURE_MSR_SPEC_CTRL);                       
 873         }                                                                        

arch/x86/kvm/cpuid.c:

 550         /*                                                                       
 551          * AMD has separate bits for each SPEC_CTRL bit.                         
 552          * arch/x86/kernel/cpu/bugs.c is kind enough to                          
 553          * record that in cpufeatures so use them.                               
 554          */                                                                      
 555         if (boot_cpu_has(X86_FEATURE_IBPB))                                      
 556                 kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB);                           
 557         if (boot_cpu_has(X86_FEATURE_IBRS))                                      
 558                 kvm_cpu_cap_set(X86_FEATURE_AMD_IBRS);                           
```

As a result, we're setting AMD-specific bits in the relevant CPUID leaf. They're
reserved, but it appears, somewhat unfortunately, that libvirt happily reports
them regardless of vendor - with the knock-on effect that anything using `virsh
capabilities` to decide which flags to pass to qemu inside the guest VM
breaks[1].

Curious if other people have hit this, and if there's specific reason why we're
setting AMD-specific flags on Intel like this.

thanks
john

[1] yes, it should be using `virsh domcapabilities`, but that didn't use to
exist AFAIK

Re: X86_FEATURE_AMD_IBRS getting set for Intel guests

[Jim Mattson]

On Mon, Jan 10, 2022 at 6:20 AM John Levon <levon@movementarian.org> wrote:
>
>
> ```
> arch/x86/kernel/cpu/common.c:
>
>  863         /*
>  864          * The Intel SPEC_CTRL CPUID bit implies IBRS and IBPB support,
>  865          * and they also have a different bit for STIBP support. Also,
>  866          * a hypervisor might have set the individual AMD bits even on
>  867          * Intel CPUs, for finer-grained selection of what's available.
>  868          */
>  869         if (cpu_has(c, X86_FEATURE_SPEC_CTRL)) {
>  870                 set_cpu_cap(c, X86_FEATURE_IBRS);
>  871                 set_cpu_cap(c, X86_FEATURE_IBPB);
>  872                 set_cpu_cap(c, X86_FEATURE_MSR_SPEC_CTRL);
>  873         }
>
> arch/x86/kvm/cpuid.c:
>
>  550         /*
>  551          * AMD has separate bits for each SPEC_CTRL bit.
>  552          * arch/x86/kernel/cpu/bugs.c is kind enough to
>  553          * record that in cpufeatures so use them.
>  554          */
>  555         if (boot_cpu_has(X86_FEATURE_IBPB))
>  556                 kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB);
>  557         if (boot_cpu_has(X86_FEATURE_IBRS))
>  558                 kvm_cpu_cap_set(X86_FEATURE_AMD_IBRS);
> ```
>
> As a result, we're setting AMD-specific bits in the relevant CPUID leaf. They're
> reserved, but it appears, somewhat unfortunately, that libvirt happily reports
> them regardless of vendor - with the knock-on effect that anything using `virsh
> capabilities` to decide which flags to pass to qemu inside the guest VM
> breaks[1].
>
> Curious if other people have hit this, and if there's specific reason why we're
> setting AMD-specific flags on Intel like this.
See the discussion at
https://patchwork.kernel.org/project/kvm/patch/1565854883-27019-2-git-send-email-pbonzini@redhat.com/.