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

[Paul Durrant]

On 21/11/2023 22:35, David Woodhouse wrote:
> On Tue, 2023-11-21 at 18:02 +0000, Paul Durrant wrote:
> > @@ -242,8 +242,7 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa,
> >          }
> >   
> >          old_pfn = gpc->pfn;
> > -       old_khva = gpc->khva - offset_in_page(gpc->khva);
> > -       old_uhva = gpc->uhva;
> > +       old_khva = (void *)PAGE_ALIGN_DOWN((uintptr_t)gpc->khva);
> >   
> >          /* If the userspace HVA is invalid, refresh that first */
> >          if (gpc->gpa != gpa || gpc->generation != slots->generation ||
> > @@ -259,13 +258,25 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa,
> >                          ret = -EFAULT;
> >                          goto out;
> >                  }
>
>
> There's a subtle behaviour change here, isn't there? I'd *really* like
> you do say 'No functional change intended' where that is true, and then
> the absence of that sentence in this one would be meaningful.
>
> You are now calling hva_to_pfn_retry() even when the uhva page hasn't
> changed. Which is harmless and probably not important, but IIUC fixable
> by the addition of:
>
>   +              if (gpc->uhva != PAGE_ALIGN_DOWN(old_uhva))

True; I can keep that optimization and then I will indeed add 'no 
functional change'... Didn't seem worth it at the time, but no harm.

> > +               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.
> > +                */
> > +               gpc->uhva = PAGE_ALIGN_DOWN(gpc->uhva);
> >          }
> >   
> > +       /* Note: the offset must be correct before calling hva_to_pfn_retry() */
> > +       gpc->uhva += page_offset;
> > +
> >          /*
> >           * If the userspace HVA changed or the PFN was already invalid,
> >           * drop the lock and do the HVA to PFN lookup again.
> >           */
> > -       if (!gpc->valid || old_uhva != gpc->uhva) {
> > +       if (!gpc->valid || hva_change) {
> >                  ret = hva_to_pfn_retry(gpc);
> >          } else {
> >                  /*
> > --
>
> But I don't really think it's that important if you can come up with a
> coherent justification for the change and note it in the commit
> message. So either way:
>
> Reviewed-by: David Woodhouse <dwmw@xxxxxxxxxxxx>

Thanks,

  Paul