Re: [PATCH 03/22] KVM: x86: Support IBPB_BRTYPE and SBPB

[Nikolay Borisov]

On 21.08.23 г. 19:35 ч., Sean Christopherson wrote:
> On Mon, Aug 21, 2023, Josh Poimboeuf wrote:
> > On Mon, Aug 21, 2023 at 10:34:38AM +0100, Andrew Cooper wrote:
> > > On 21/08/2023 2:19 am, Josh Poimboeuf wrote:
> > > > The IBPB_BRTYPE and SBPB CPUID bits aren't set by HW.
> > >
> > > "Current hardware".
> > >
> > > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > > index c381770bcbf1..dd7472121142 100644
> > > > --- a/arch/x86/kvm/x86.c
> > > > +++ b/arch/x86/kvm/x86.c
> > > > @@ -3676,12 +3676,13 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> > > >   		if (!msr_info->host_initiated && !guest_has_pred_cmd_msr(vcpu))
> > > >   			return 1;
> > > >   
> > > > -		if (!boot_cpu_has(X86_FEATURE_IBPB) || (data & ~PRED_CMD_IBPB))
> > > > +		if (boot_cpu_has(X86_FEATURE_IBPB) && data == PRED_CMD_IBPB)
> > > > +			wrmsrl(MSR_IA32_PRED_CMD, PRED_CMD_IBPB);
> > > > +		else if (boot_cpu_has(X86_FEATURE_SBPB) && data == PRED_CMD_SBPB)
> > > > +			wrmsrl(MSR_IA32_PRED_CMD, PRED_CMD_SBPB);
> > > > +		else if (data)
> > > >   			return 1;
> > >
> > > SBPB | IBPB is an explicitly permitted combination, but will be rejected
> > > by this logic.
> >
> > Ah yes, I see that now:
> >
> >    If software writes PRED_CMD with both bits 0 and 7 set to 1, the
> >    processor performs an IBPB operation.
>
> The KVM code being a bit funky isn't doing you any favors.  This is the least
> awful approach I could come up with:
>
> 	case MSR_IA32_PRED_CMD: {
> 		u64 reserved_bits = ~(PRED_CMD_IBPB | PRED_CMD_SBPB);
>
> 		if (!msr_info->host_initiated) {
> 			if (!guest_has_pred_cmd_msr(vcpu))
> 				return 1;
>
> 			if (!guest_cpuid_has(vcpu, X86_FEATURE_SBPB))
> 				reserved_bits |= PRED_CMD_SBPB;
> 		}
>
> 		if (!boot_cpu_has(X86_FEATURE_IBPB))
> 			reserved_bits |= PRED_CMD_IBPB;
>
> 		if (!boot_cpu_has(X86_FEATURE_SBPB))
> 			reserved_bits |= PRED_CMD_SBPB;
>
> 		if (!data)
> 			break;
>
> 		wrmsrl(MSR_IA32_PRED_CMD, data);
> 		break;
> 	}
>
> There are more wrinkles though.  KVM passes through MSR_IA32_PRED_CMD based on
> IBPB.  If hardware supports both IBPB and SBPB, but userspace does NOT exposes
> SBPB to the guest, then KVM will create a virtualization hole where the guest can
> write SBPB against userspace's wishes.  I haven't read up on SBPB enought o know
> whether or not that's problematic.
>
> And conversely, if userspace expoes SBPB but not IBPB, then KVM will intercept
> writes to MSR_IA32_PRED_CMD and probably tank guest performance.  Again, I haven't
> paid attention enough to know if this is a reasonable configuration, i.e. whether
> or not it's worth caring about in KVM.
>
> If the virtualization holes are deemed safe, then the easiest thing would be to
> treat MSR_IA32_PRED_CMD as existing if either IBPB or SBPB exists.  E.g.
>
> diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
> index b1658c0de847..e4db844a58fe 100644
> --- a/arch/x86/kvm/cpuid.h
> +++ b/arch/x86/kvm/cpuid.h
> @@ -174,7 +174,8 @@ static inline bool guest_has_spec_ctrl_msr(struct kvm_vcpu *vcpu)
>   static inline bool guest_has_pred_cmd_msr(struct kvm_vcpu *vcpu)
>   {
>          return (guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL) ||
> -               guest_cpuid_has(vcpu, X86_FEATURE_AMD_IBPB));
> +               guest_cpuid_has(vcpu, X86_FEATURE_AMD_IBPB) ||
> +               guest_cpuid_has(vcpu, X86_FEATURE_SBPB));
>   }
>   
>   static inline bool supports_cpuid_fault(struct kvm_vcpu *vcpu)
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 12688754c556..aa4620fb43f8 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3656,17 +3656,33 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>                  vcpu->arch.perf_capabilities = data;
>                  kvm_pmu_refresh(vcpu);
>                  break;
> -       case MSR_IA32_PRED_CMD:
> -               if (!msr_info->host_initiated && !guest_has_pred_cmd_msr(vcpu))
> -                       return 1;
> +       case MSR_IA32_PRED_CMD: {
> +               u64 reserved_bits = ~(PRED_CMD_IBPB | PRED_CMD_SBPB);
> +
> +               if (!msr_info->host_initiated) {
> +                       if (!guest_has_pred_cmd_msr(vcpu))
> +                               return 1;
> +
> +                       if (!guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL) &&
> +                           !guest_cpuid_has(vcpu, X86_FEATURE_AMD_IBPB))
> +                               reserved_bits |= PRED_CMD_IBPB;
> +
> +                       if (!guest_cpuid_has(vcpu, X86_FEATURE_SBPB))
> +                               reserved_bits |= PRED_CMD_SBPB;
> +               }
> +
> +               if (!boot_cpu_has(X86_FEATURE_IBPB))
> +                       reserved_bits |= PRED_CMD_IBPB;
> +
> +               if (!boot_cpu_has(X86_FEATURE_SBPB))
> +                       reserved_bits |= PRED_CMD_SBPB;
>   
> -               if (!boot_cpu_has(X86_FEATURE_IBPB) || (data & ~PRED_CMD_IBPB))
> -                       return 1;


Surely data must be sanitized against reserved_bit before this if is 
executed?

>                  if (!data)
>                          break;
>   
> -               wrmsrl(MSR_IA32_PRED_CMD, PRED_CMD_IBPB);
> +               wrmsrl(MSR_IA32_PRED_CMD, data);
>                  break;
> +       }
>          case MSR_IA32_FLUSH_CMD:
>                  if (!msr_info->host_initiated &&
>                      !guest_cpuid_has(vcpu, X86_FEATURE_FLUSH_L1D))