linux-kernel - Re: [PATCH v2] KVM: SEV-ES: Don't intercept MSR_IA32

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Date: Mon, 20 May 2024 10:36:44 +0530
From: Ravi Bangoria <ravi.bangoria@....com>
To: Sean Christopherson <seanjc@...gle.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,
 Ravi Bangoria <ravi.bangoria@....com>
Subject: Re: [PATCH v2] KVM: SEV-ES: Don't intercept MSR_IA32_DEBUGCTLMSR for
 SEV-ES guests

On 20-May-24 10:34 AM, 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@....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
> a Genoa machine. Also, LBR MSRs (except MSR_AMD_DBG_EXTN_CFG) are of swap type

I meant LBR Stack MSRs.