Re: [PATCH v8 07/15] KVM: pfncache: include page offset in uhva and use it consistently

[Paul Durrant]

On 22/11/2023 15:42, David Woodhouse wrote:
> On Wed, 2023-11-22 at 22:27 +0800, Xu Yilun wrote:
> > On Wed, Nov 22, 2023 at 09:12:18AM +0000, David Woodhouse wrote:
> > > On Wed, 2023-11-22 at 16:54 +0800, Xu Yilun wrote:
> > > > > @@ -259,13 +258,25 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa,
> > > > >                          ret = -EFAULT;
> > > > >                          goto out;
> > > > >                  }
> > > > > +
> > > > > +               hva_change = true;
> > > > > +       } else {
> > > > > +               /*
> > > > > +                * No need to do any re-mapping if the only thing that has
> > > > > +                * changed is the page offset. Just page align it to allow the
> > > > > +                * new offset to be added in.
> > > >
> > > > I don't understand how the uhva('s offset) could be changed when both gpa and
> > > > slot are not changed. Maybe I have no knowledge of xen, but in later
> > > > patch you said your uhva would never change...
> > >
> > > It doesn't change on a normal refresh with kvm_gpc_refresh(), which is
> > > just for revalidation after memslot changes or MMU invalidation.
> > >
> > > But it can change if the gpc is being reinitialized with a new address
> > > (perhaps because the guest has made another hypercall to set the
> > > address, etc.)
> > >
> > > That new address could happen to be in the *same* page as the previous
> >
> > In this case, the lower bits of new gpa should be different to gpc->gpa,
> > so will hit "if (gpc->gpa != gpa ...)" branch.
>
> I think that 'if (gpc->gpa != gpa); branch is also gratuitously
> refreshing when it doesn't need to; it only needs to refresh if the
> *aligned* gpas don't match.

I did look at that but decided that gfn_to_hva_memslot() was 
sufficiently lightweight that it was not really worth optimising.

> But it was like that already, so I won't heckle Paul any further :)

I appreciate it! :-)

  Paul