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.
