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@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 I meant LBR Stack MSRs.
