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

[Ravi Bangoria]

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@xxxxxxx>
>>>   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
C so this overhead is only for guest MSR save/restore.

* The overhead was measured using instrumentation code, it's not an official
  number provided by hw folks.

Thanks,
Ravi