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

[Sean Christopherson <seanjc@google.com>]

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.