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Message-ID: <ZkdqW8JGCrUUO3RA@google.com>
Date: Fri, 17 May 2024 07:31:55 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Ravi Bangoria <ravi.bangoria@....com>
Cc: pbonzini@...hat.com, thomas.lendacky@....com, tglx@...utronix.de,
mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
hpa@...or.com, michael.roth@....com, nikunj.dadhania@....com,
kvm@...r.kernel.org, linux-kernel@...r.kernel.org, santosh.shukla@....com
Subject: Re: [PATCH v2] KVM: SEV-ES: Don't intercept MSR_IA32_DEBUGCTLMSR for
SEV-ES guests
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@....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.
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