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
