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

[Sean Christopherson]

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?

> 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.