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

[Ravi Bangoria]

On 5/22/2024 3:52 AM, Sean Christopherson wrote:
> On Tue, May 21, 2024, Paolo Bonzini wrote:
>> On Tue, May 21, 2024 at 10:31 PM Sean Christopherson <seanjc@xxxxxxxxxx> 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@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
>>>
>>> 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.

Sure, will keep it. I'll respin with all these feedback addressed.

Thanks,
Ravi