Re: [PATCH 1/2] KVM: nVMX: fix CR4_READ_SHADOW when L0 updates CR4 during a signal

Thomas Prescher
2024-04-18T13:48:30Z

On Wed, 2024-04-17 at 09:28 -0700, Sean Christopherson wrote:
> On Wed, Apr 17, 2024, Sean Christopherson wrote:
> > On Wed, Apr 17, 2024, Thomas Prescher wrote:
> > > On Tue, 2024-04-16 at 11:07 -0700, Sean Christopherson wrote:
> > > > Hur dur, I forgot that KVM provides a "guest_mode" stat.
> > > > Userspace can do
> > > > KVM_GET_STATS_FD on the vCPU FD to get a file handle to the
> > > > binary stats,
> > > > and then you wouldn't need to call back into KVM just to query
> > > > guest_mode.
> > > >
> > > > Ah, and I also forgot that we have kvm_run.flags, so adding
> > > > KVM_RUN_X86_GUEST_MODE would also be trivial (I almost
> > > > suggested it
> > > > earlier, but didn't want to add a new field to kvm_run without
> > > > a very good
> > > > reason).
> > >
> > > Thanks for the pointers. This is really helpful.
> > >
> > > I tried the "guest_mode" stat as you suggested and it solves the
> > > immediate issue we have with VirtualBox/KVM.
> >
> > Note,
>
> Gah, got distracted.  I was going to say that we should add
> KVM_RUN_X86_GUEST_MODE,
> because stats aren't guaranteed ABI[*], i.e. relying on guest_mode
> could prove
> problematic in the long run (though that's unlikely).
>
> [*]
> https://lore.kernel.org/all/CABgObfZ4kqaXLaOAOj4aGB5GAe9GxOmJmOP+7kdke6OqA35HzA@mail.gmail.com

Allright. I will propose a patch that sets the KVM_RUN_X86_GUEST_MODE
flag in the next couple of days. Do we also need a new capability for
this flag so userland can query whether this field is actually updated
by KVM?