Re: [PATCH] KVM: nVMX: Hide shadow VMCS right after VMCLEAR

[Sean Christopherson <seanjc@google.com>]

On Tue, Jul 21, 2026, Paolo Bonzini wrote:
> On Tue, Jul 21, 2026 at 5:25 PM Sean Christopherson <seanjc@google.com> wrote:
> > FWIW, I had a pretty strong preference for Paolo's off-list suggestion of
> > explicitly disabling preemption, even before seeing Sashiko's response.  Every
> > other path the VMCLEARs the shadow VMCS runs with preemption disabled, IMO it's
> > more intuitive to follow suit here.  And even with the comment, it took a good
> > 5-10 minutes of staring to understand what exactly is going wrong, though that's
> > probably a me problem.
> 
> Well, it would be at least an us problem---but I thought I was in the
> minority... The only (IMO bad) reason to leave preemption enabled is
> to obscure the change, but that's a lost battle anyway these days so
> why bother.
> 
> That said, I am surprised that VMCLEAR/VMREAD/VMWRITE/VMPTRLD do not
> have a memory clobber, since they access internal processor registers
> and the compiler may reorder them without the clobber. Fixing that
> would also work around Sashiko's complaint, so I guess we should do it
> anyway? 

Ya, good point.  We should probably hardcode that into vmx_asm1() and vmx_asm2()?
INVVPID and INVEPT also need the "memory" clobber, no?

> And disabling preemption can be done separately on top of this change, which
> is already on its way to Linus.

+1