Re: [PATCH v8 15/15] KVM: x86/cpuid: Advise Arch LBR feature in CPUID

[Like Xu]

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.

The KVM LBR implementation may not break it until additional support is applied 
on the host side.

We'd better not let the guest down if the user space specifies fewer or more LBRs,
and explicitly rejecting it in the CPUID settings is an option, or just let the 
error happen.

and KVM should darn well verify that any MSRs it's exposing to the
> > guest actually exist.
> Hi, Sean,
> Thanks for the comments!
> The treatment for LBR depth is a bit special, only the host value can be
> supported now, i.e., 32. If userspace set the value other that 32, would like
> to notify it as early as possible.
> Do you want to remove the check here and correct the invalid setting silently when
> guest is querying CPUID?
>
> > > +		cpuid_count(0x1c, 0, &eax, &ebx, &ecx, &edx);
> > > +
> > > +		if ((best->eax & 0xff) != BIT(fls(eax & 0xff) - 1))
> > > +			return -EINVAL;
> > > +	}
> > >   
> > >   	return 0;
> > >   }