Re: [PATCH v2] KVM: SEV-ES: Don't intercept MSR_IA32_DEBUGCTLMSR for SEV-ES guests

[Sean Christopherson <seanjc@google.com>]

On Tue, May 21, 2024, Paolo Bonzini wrote:
> On Tue, May 21, 2024 at 10:31 PM Sean Christopherson <seanjc@google.com> wrote:
> >
> > On Mon, May 20, 2024, Ravi Bangoria wrote:
> > > On 17-May-24 8:01 PM, Sean Christopherson wrote:
> > > > On Fri, May 17, 2024, Ravi Bangoria wrote:
> > > >> On 08-May-24 12:37 AM, Sean Christopherson wrote:
> > > >>> So unless I'm missing something, the only reason to ever disable LBRV would be
> > > >>> for performance reasons.  Indeed the original commits more or less says as much:
> > > >>>
> > > >>>   commit 24e09cbf480a72f9c952af4ca77b159503dca44b
> > > >>>   Author:     Joerg Roedel <joerg.roedel@amd.com>
> > > >>>   AuthorDate: Wed Feb 13 18:58:47 2008 +0100
> > > >>>
> > > >>>     KVM: SVM: enable LBR virtualization
> > > >>>
> > > >>>     This patch implements the Last Branch Record Virtualization (LBRV) feature of
> > > >>>     the AMD Barcelona and Phenom processors into the kvm-amd module. It will only
> > > >>>     be enabled if the guest enables last branch recording in the DEBUG_CTL MSR. So
> > > >>>     there is no increased world switch overhead when the guest doesn't use these
> > > >>>     MSRs.
> > > >>>
> > > >>> but what it _doesn't_ say is what the world switch overhead is when LBRV is
> > > >>> enabled.  If the overhead is small, e.g. 20 cycles?, then I see no reason to
> > > >>> keep the dynamically toggling.
> > > >>>
> > > >>> And if we ditch the dynamic toggling, then this patch is unnecessary to fix the
> > > >>> LBRV issue.  It _is_ necessary to actually let the guest use the LBRs, but that's
> > > >>> a wildly different changelog and justification.
> > > >>
> > > >> The overhead might be less for legacy LBR. But upcoming hw also supports
> > > >> LBR Stack Virtualization[1]. LBR Stack has total 34 MSRs (two control and
> > > >> 16*2 stack). Also, Legacy and Stack LBR virtualization both are controlled
> > > >> through the same VMCB bit. So I think I still need to keep the dynamic
> > > >> toggling for LBR Stack virtualization.
> > > >
> > > > Please get performance number so that we can make an informed decision.  I don't
> > > > want to carry complexity because we _think_ the overhead would be too high.
> > >
> > > LBR Virtualization overhead for guest entry + exit roundtrip is ~450 cycles* on
> >
> > Ouch.  Just to clearify, that's for LBR Stack Virtualization, correct?
> 
> And they are all in the VMSA, triggered by LBR_CTL_ENABLE_MASK, for
> non SEV-ES guests?
> 
> > Anyways, I agree that we need to keep the dynamic toggling.
> > But I still think we should delete the "lbrv" module param.  LBR Stack support has
> > a CPUID feature flag, i.e. userspace can disable LBR support via CPUID in order
> > to avoid the overhead on CPUs with LBR Stack.
> 
> The "lbrv" module parameter is only there to test the logic for
> processors (including nested virt) that don't have LBR virtualization.
> But the only effect it has is to drop writes to
> MSR_IA32_DEBUGCTL_MSR...
> 
> >                 if (kvm_cpu_cap_has(X86_FEATURE_LBR_STACK) &&
> >                     !guest_cpuid_has(vcpu, X86_FEATURE_LBR_STACK)) {
> >                         kvm_pr_unimpl_wrmsr(vcpu, ecx, data);
> >                         break;
> >                 }
> 
> ... and if you have this, adding an "!lbrv ||" is not a big deal, and
> allows testing the code on machines without LBR stack.

Yeah, but keeping lbrv also requires tying KVM's X86_FEATURE_LBR_STACK capability
to lbrv, i.e. KVM shouldn't advetise X86_FEATURE_LBR_STACK if lbrv=false.  And
KVM needs to condition SEV-ES on lbrv=true.  Neither of those are difficult to
handle, e.g. svm_set_cpu_caps() already checks plenty of module params, I'm just
not convinced legacy LRB virtualization is interesting enough to warrant a module
param.

That said, I'm ok keeping the param if folks prefer that approach.