On 5/22/2024 2:01 AM, Sean Christopherson 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? Includes both, since there is a single enable bit shared by them. > Ugh, I was going to say that we could always enable "legacy" LBR virtualization, > and do the dynamic toggling iff DebugExtnCtl.LBRS=1, but they share an enabling > flag. What a mess. Agreed. can't help :( >> 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. > > Lovely. > > Have I mentioned that the SEV-ES behavior of force-enabling every feature under > the sun is really, really annoying? > > Anyways, I agree that we need to keep the dynamic toggling. Sure. Will prepare v3 accordingly. Thanks, Ravi
