Re: [PATCH v8 08/15] KVM: pfncache: allow a cache to be activated with a fixed (userspace) HVA

[Paul Durrant]

On 21/11/2023 22:47, David Woodhouse wrote:
> On Tue, 2023-11-21 at 18:02 +0000, Paul Durrant wrote:
> > -static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa,
> > +static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, u64 addr, bool addr_is_gpa,
> >                               unsigned long len)
> >   {
> >          struct kvm_memslots *slots = kvm_memslots(gpc->kvm);
> > -       unsigned long page_offset = offset_in_page(gpa);
> > +       unsigned long page_offset = offset_in_page(addr);
> >          bool unmap_old = false;
> >          kvm_pfn_t old_pfn;
> >          bool hva_change = false;
> > @@ -244,12 +244,21 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa,
> >          old_pfn = gpc->pfn;
> >          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 ||
> > -           kvm_is_error_hva(gpc->uhva)) {
> > -               gfn_t gfn = gpa_to_gfn(gpa);
> > +       if (!addr_is_gpa) {
> > +               gpc->gpa = KVM_XEN_INVALID_GPA;
> > +               gpc->uhva = PAGE_ALIGN_DOWN(gpc->uhva);
> > +               addr = PAGE_ALIGN_DOWN(addr);
> > +
> > +               if (gpc->uhva != addr) {
> > +                       gpc->uhva = addr;
> > +                       hva_change = true;
> > +               }
> > +       } else if (gpc->gpa != addr ||
> > +                  gpc->generation != slots->generation ||
> > +                  kvm_is_error_hva(gpc->uhva)) {
> > +               gfn_t gfn = gpa_to_gfn(addr);
> >   
> > -               gpc->gpa = gpa;
> > +               gpc->gpa = addr;
> >                  gpc->generation = slots->generation;
> >                  gpc->memslot = __gfn_to_memslot(slots, gfn);
> >                  gpc->uhva = gfn_to_hva_memslot(gpc->memslot, gfn);
>
> Hrm, now that a previous patch means we're preserving the low bits of
> gpc->uhva surely you don't *need* to mess with the gpc struct?

I'm not messing with it, am I?

> If gpc->gpa == KVM_XEN_INVALID_GPA (but gpc->uhva != KVM_ERR_ERR_BAD &&
> gpc->active) surely that's enough to signal that gpc->uhva is canonical
> and doesn't need to be looked up from the GPA?
>
> And I think that means the 'bool addr_is_gpa' argument can go away from
> __kvm_gpc_refresh(); you can set it up in {__,}kvm_gpc_activate*()
> instead?

Alas not... __kvm_gpc_refresh() still needs to know *something* has 
changed, otherwise the khva will be stale.

  Paul