Re: [PATCH 2/8] KVM: pfncache: add a mark-dirty helper

[Paul Durrant]

On 14/09/2023 10:21, David Woodhouse wrote:
> On Thu, 2023-09-14 at 08:49 +0000, Paul Durrant wrote:
> > --- a/arch/x86/kvm/xen.c
> > +++ b/arch/x86/kvm/xen.c
> > @@ -430,14 +430,13 @@ static void kvm_xen_update_runstate_guest(struct kvm_vcpu *v, bool atomic)
> >                  smp_wmb();
> >          }
> >   
> > -       if (user_len2)
> > +       if (user_len2) {
> > +               kvm_gpc_mark_dirty(gpc2);
> >                  read_unlock(&gpc2->lock);
> > +       }
> >   
> > +       kvm_gpc_mark_dirty(gpc1);
> >          read_unlock_irqrestore(&gpc1->lock, flags);
> > -
> > -       mark_page_dirty_in_slot(v->kvm, gpc1->memslot, gpc1->gpa >> PAGE_SHIFT);
> > -       if (user_len2)
> > -               mark_page_dirty_in_slot(v->kvm, gpc2->memslot, gpc2->gpa >> PAGE_SHIFT);
> >   }
> >   
> >   void kvm_xen_update_runstate(struct kvm_vcpu *v, int state)
>
> ISTR there was a reason why the mark_page_dirty_in_slot() was called
> *after* unlocking. Although now I say it, that seems wrong... is that
> because the spinlock is only protecting the uHVA→kHVA mapping, while
> the memslot/gpa are going to remain valid even after unlock, because
> those are protected by sRCU?

Without the lock you could see an inconsistent GPA and memslot so I 
think you could theoretically calculate a bogus rel_gfn and walk off the 
end of the dirty bitmap. Hence moving the call inside the lock while I 
was in the neighbourhood seemed like a good idea. I could call it out in 
the commit comment if you'd like.

  Paul