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

[Sean Christopherson]

On Mon, May 17, 2021, Paolo Bonzini wrote:
> On 14/05/21 07:11, Andy Lutomirski wrote:
> > I don't even want to think about what happens if a perf NMI hits and
> > accesses host user memory while the guest PKRU is live (on VMX -- I
> > think this can't happen on SVM).
> 
> This is indeed a problem, which indeed cannot happen on SVM but is there on
> VMX.  Note that the function above is not handling all of the xstate, it's
> handling the *XSAVE state*, that is XCR0, XSS and PKRU.  Thus the window is
> small, but it's there.
> 
> Is it solvable at all, without having PKRU fields in the VMCS (and without
> masking NMIs in the LAPIC which would be too expensive)?  Dave, Sean, what
> do you think?

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.