On Tue, Jul 13, 2021 at 2:49 AM Like Xu <like.xu.linux@xxxxxxxxx> wrote: > > On 13/7/2021 1:45 am, Jim Mattson wrote: > > On Mon, Jul 12, 2021 at 10:20 AM Jim Mattson <jmattson@xxxxxxxxxx> wrote: > >> > >> On Mon, Jul 12, 2021 at 3:19 AM Like Xu <like.xu.linux@xxxxxxxxx> wrote: > >>> > >>> On 12/7/2021 5:53 pm, Yang Weijiang wrote: > >>>> On Fri, Jul 09, 2021 at 04:41:30PM -0700, Jim Mattson wrote: > >>>>> On Fri, Jul 9, 2021 at 3:54 PM Jim Mattson <jmattson@xxxxxxxxxx> wrote: > >>>>>> > >>>>>> On Fri, Jul 9, 2021 at 2:51 AM Yang Weijiang <weijiang.yang@xxxxxxxxx> wrote: > >>>>>>> > >>>>>>> If host is using MSR_ARCH_LBR_CTL then save it before vm-entry > >>>>>>> and reload it after vm-exit. > >>>>>> > >>>>>> I don't see anything being done here "before VM-entry" or "after > >>>>>> VM-exit." This code seems to be invoked on vcpu_load and vcpu_put. > >>>>>> > >>>>>> In any case, I don't see why this one MSR is special. It seems that if > >>>>>> the host is using the architectural LBR MSRs, then *all* of the host > >>>>>> architectural LBR MSRs have to be saved on vcpu_load and restored on > >>>>>> vcpu_put. Shouldn't kvm_load_guest_fpu() and kvm_put_guest_fpu() do > >>>>>> that via the calls to kvm_save_current_fpu(vcpu->arch.user_fpu) and > >>>>>> restore_fpregs_from_fpstate(&vcpu->arch.user_fpu->state)? > >>>>> > >>>>> It does seem like there is something special about IA32_LBR_DEPTH, though... > >>>>> > >>>>> Section 7.3.1 of the Intel® Architecture Instruction Set Extensions > >>>>> and Future Features Programming Reference > >>>>> says, "IA32_LBR_DEPTH is saved by XSAVES, but it is not written by > >>>>> XRSTORS in any circumstance." It seems like that would require some > >>>>> special handling if the host depth and the guest depth do not match. > >>>> In our vPMU design, guest depth is alway kept the same as that of host, > >>>> so this won't be a problem. But I'll double check the code again, thanks! > >>> > >>> KVM only exposes the host's depth value to the user space > >>> so the guest can only use the same depth as the host. > >> > >> The allowed depth supplied by KVM_GET_SUPPORTED_CPUID isn't enforced, > >> though, is it? > > Like other hardware dependent features, the functionality will not > promise to work properly if the guest uses the unsupported CPUID. It's fine if it doesn't work in the guest, but can't a guest with the wrong depth prevent the host LBRs from being reloaded when switching back to the host state? It's definitely not okay for an ill-configured guest to break host functionality. > > > > Also, doesn't this end up being a major constraint on future > > platforms? Every host that this vCPU will ever run on will have to use > > the same LBR depth as the host on which it was started. > > > > As a first step, we made the guest LBR feature only available for the > "migratable=off" user space, which is why we intentionally did not add > MSR_ARCH_LBR_* stuff to msrs_to_save_all[] in earlier versions. We have no such concept in our user space. Features that are not migratable should clearly be identified as such by an appropriate KVM API. At present, I don't believe there is such an API. > But hopefully, we may make it at least migratable for Arch LBR. > > I'm personally curious about the cost of using XSAVES to swicth > guest lbr msrs during vmx transaction, and if the cost is unacceptable, > we may ask the perf host to adjust different depths for threads. > >
