Re: [PATCH] KVM: x86: add hint to skip hidden rdpkru under kvm_load_host_xsave_state

[Sean Christopherson]

On Mon, May 17, 2021, Jim Mattson wrote:
> On Mon, May 17, 2021 at 11:05 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> >
> > On Mon, May 17, 2021, Dave Hansen wrote:
> > > On 5/17/21 10:49 AM, Andy Lutomirski wrote:
> > > >> The least awful solution would be to have the NMI handler restore
> > > >> the host's PKRU.  The NMI handler would need to save/restore the
> > > >> register, a la CR2, but the whole thing could be optimized to run
> > > >> if and only if the NMI lands in the window where the guest's PKRU
> > > >> is loaded.
> > > >
> > > > Or set a flag causing nmi_uaccess_ok() to return false.
> > >
> > > Oh, that doesn't sound too bad.  The VMENTER/EXIT paths are also
> > > essentially a context switch.
> >
> > I like that idea, too.
> >
> > The flag might also be useful to fix the issue where the NMI handler activates
> > PEBS after KVM disables it.  Jim?
> 
> The issue is actually that the NMI handler *clears* IA32_PEBS_ENABLE
> bits after giving out the host value of the MSR to KVM. If we were to
> block the NMI handler from modifying IA32_PEBS_ENABLE until after the
> next VM-exit, that could solve this issue. I don't know if it makes
> sense to piggyback on nmi_uaccess(), though.

I wasn't thinking about using nmi_uaccess_okay(), but rather whatever flag is
added so that can KVM can inform the NMI handler that KVM is in the middle of
its version of a context switch.

> > > Will widening the window where nmi_uaccess_okay()==false anger any of
> > > the perf folks?  It looks like perf knows how to handle it nicely.