On Fri, Oct 15, 2021, Like Xu wrote:
> On 15/10/2021 9:28 am, Yang Weijiang wrote:
> > On Fri, Oct 15, 2021 at 12:01:22AM +0000, Sean Christopherson wrote:
> > > s/Advise/Advertise
> > >
> > > On Tue, Aug 24, 2021, Yang Weijiang wrote:
> > > > Add Arch LBR feature bit in CPU cap-mask to expose the feature.
> > > > Only max LBR depth is supported for guest, and it's consistent
> > > > with host Arch LBR settings.
> > > >
> > > > Co-developed-by: Like Xu <like.xu@xxxxxxxxxxxxxxx>
> > > > Signed-off-by: Like Xu <like.xu@xxxxxxxxxxxxxxx>
> > > > Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> > > > ---
> > > > arch/x86/kvm/cpuid.c | 33 ++++++++++++++++++++++++++++++++-
> > > > 1 file changed, 32 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > > > index 03025eea1524..d98ebefd5d72 100644
> > > > --- a/arch/x86/kvm/cpuid.c
> > > > +++ b/arch/x86/kvm/cpuid.c
> > > > @@ -88,6 +88,16 @@ static int kvm_check_cpuid(struct kvm_cpuid_entry2 *entries, int nent)
> > > > if (vaddr_bits != 48 && vaddr_bits != 57 && vaddr_bits != 0)
> > > > return -EINVAL;
> > > > }
> > > > + best = cpuid_entry2_find(entries, nent, 0x1c, 0);
> > > > + if (best) {
> > > > + unsigned int eax, ebx, ecx, edx;
> > > > +
> > > > + /* Reject user-space CPUID if depth is different from host's.*/
> > >
> > > Why disallow this? I don't see why it would be illegal for userspace to specify
> > > fewer LBRs,
>
> The emulation of guest LBR *depends* on the host LBR driver to save/restore
> LBRs entries (which are pass-through to the guest and store the guest branch
> instructions rips actually).
>
> Currently, the host side does not support the use of different lbr depths on
> the same host to customize this part of the overhead. The host perf LBR
> driver assumes that the lbr depths of different tasks on different cpu's are
> the same and are the maximum value.
Yes, I assumed as much, but saving/restoring MSRs that the guest does not have
access to is not a functional issue, just a waste of cycles.
