On Fri, Jul 09, 2021 at 03:54:53PM -0700, Jim Mattson 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)? I looked back on the discussion thread: https://patchwork.kernel.org/project/kvm/patch/20210303135756.1546253-8-like.xu@xxxxxxxxxxxxxxx/ not sure why this code is added, but IMO, although fpu save/restore in outer loop covers this LBR MSR, but the operation points are far away from vm-entry/exit point, i.e., the guest MSR setting could leak to host side for a signicant long of time, it may cause host side profiling accuracy. if we save/restore it manually, it'll mitigate the issue signifcantly.
