Re: [PATCH 5/6] KVM: x86: Virtualize FLUSH_L1D and passthrough MSR_IA32_FLUSH_CMD

[Zhi Wang <zhi.wang.linux@gmail.com>]

On Thu, 23 Mar 2023 15:17:50 -0700
Sean Christopherson <seanjc@google.com> wrote:

> On Wed, Mar 22, 2023, Pawan Gupta wrote:
> > On Tue, Mar 21, 2023 at 06:14:39PM -0700, Sean Christopherson wrote:
> > > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > > index 85bb535fc321..b32edaf5a74b 100644
> > > --- a/arch/x86/kvm/svm/svm.c
> > > +++ b/arch/x86/kvm/svm/svm.c
> > > @@ -95,6 +95,7 @@ static const struct svm_direct_access_msrs {
> > >  #endif
> > >  	{ .index = MSR_IA32_SPEC_CTRL,			.always = false },
> > >  	{ .index = MSR_IA32_PRED_CMD,			.always = false },
> > > +	{ .index = MSR_IA32_FLUSH_CMD,			.always = false },
> > >  	{ .index = MSR_IA32_LASTBRANCHFROMIP,		.always = false },
> > >  	{ .index = MSR_IA32_LASTBRANCHTOIP,		.always = false },
> > >  	{ .index = MSR_IA32_LASTINTFROMIP,		.always = false },
> > > @@ -4140,6 +4141,10 @@ static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> > >  		set_msr_interception(vcpu, svm->msrpm, MSR_IA32_PRED_CMD, 0,
> > >  				     !!guest_has_pred_cmd_msr(vcpu));
> > >  
> > > +	if (boot_cpu_has(X86_FEATURE_FLUSH_L1D))
> > 
> > Just curious, will this ever be true on AMD hardware? AFAIK,
> > X86_FEATURE_FLUSH_L1D is Intel-defined CPU feature.
> 
> Don't know myself, but I assume/home there was actual motivation behind adding
> support for AMD.

Hmm. I took a look on the APM[1] published on Jan 2023, 3.2.9 Speculation
control MSRs. It only has SPEC_CTL/PRED_SMD so far. Also, the information
here [2] shows this is a mitigation only for Intel CPUs. Looks like AMD
does not require this so far.

[1] https://www.amd.com/system/files/TechDocs/40332.pdf
[2] https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/l1tf.html