Re: [PATCH v7 2/8] KVM: Introduce __kvm_follow_pfn function

[David Stevens]

On Wed, Jul 5, 2023 at 12:10 PM Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
>
> > @@ -2514,35 +2512,26 @@ static bool hva_to_pfn_fast(unsigned long addr, bool write_fault,
> >   * The slow path to get the pfn of the specified host virtual address,
> >   * 1 indicates success, -errno is returned if error is detected.
> >   */
> > -static int hva_to_pfn_slow(unsigned long addr, bool *async, bool write_fault,
> > -                        bool interruptible, bool *writable, kvm_pfn_t *pfn)
> > +static int hva_to_pfn_slow(struct kvm_follow_pfn *foll, kvm_pfn_t *pfn)
> >  {
> > -     unsigned int flags = FOLL_HWPOISON;
> > +     unsigned int flags = FOLL_HWPOISON | FOLL_GET | foll->flags;
> >       struct page *page;
> >       int npages;
> >
> >       might_sleep();
> >
> > -     if (writable)
> > -             *writable = write_fault;
> > -
> > -     if (write_fault)
> > -             flags |= FOLL_WRITE;
> > -     if (async)
> > -             flags |= FOLL_NOWAIT;
> > -     if (interruptible)
> > -             flags |= FOLL_INTERRUPTIBLE;
> > -
> > -     npages = get_user_pages_unlocked(addr, 1, &page, flags);
> > +     npages = get_user_pages_unlocked(foll->hva, 1, &page, flags);
> >       if (npages != 1)
> >               return npages;
> >
> > +     foll->writable = (foll->flags & FOLL_WRITE) && foll->allow_write_mapping;
> > +
> >       /* map read fault as writable if possible */
> > -     if (unlikely(!write_fault) && writable) {
> > +     if (unlikely(!foll->writable) && foll->allow_write_mapping) {
>
> I guess !foll->writable should be !(foll->flags & FOLL_WRITE) here.

The two statements are logically equivalent, although I guess using
!(foll->flags & FOLL_WRITE) may be a little clearer, if a little more
verbose.

> >               struct page *wpage;
> >
> > -             if (get_user_page_fast_only(addr, FOLL_WRITE, &wpage)) {
> > -                     *writable = true;
> > +             if (get_user_page_fast_only(foll->hva, FOLL_WRITE, &wpage)) {
> > +                     foll->writable = true;
> >                       put_page(page);
> >                       page = wpage;
> >               }
> > @@ -2572,23 +2561,23 @@ static int kvm_try_get_pfn(kvm_pfn_t pfn)
> >       return get_page_unless_zero(page);
> >  }
> >
> ...
>
> > +kvm_pfn_t __gfn_to_pfn_memslot(const struct kvm_memory_slot *slot, gfn_t gfn,
> > +                            bool atomic, bool interruptible, bool *async,
> > +                            bool write_fault, bool *writable, hva_t *hva)
> > +{
> > +     kvm_pfn_t pfn;
> > +     struct kvm_follow_pfn foll = {
> > +             .slot = slot,
> > +             .gfn = gfn,
> > +             .flags = 0,
> > +             .atomic = atomic,
> > +             .allow_write_mapping = !!writable,
> > +     };
> > +
> > +     if (write_fault)
> > +             foll.flags |= FOLL_WRITE;
> > +     if (async)
> > +             foll.flags |= FOLL_NOWAIT;
> > +     if (interruptible)
> > +             foll.flags |= FOLL_INTERRUPTIBLE;
> > +
> > +     pfn = __kvm_follow_pfn(&foll);
> > +     if (pfn == KVM_PFN_ERR_NEEDS_IO) {
>
> Could we just use KVM_PFN_ERR_FAULT and foll.flags here? I.e.,
>         if (pfn == KVM_PFN_ERR_FAULT && (foll.flags & FOLL_NOWAIT))?
> Setting pfn to KVM_PFN_ERR_NEEDS_IO just to indicate an async fault
> seems unnecessary.

There are the cases where the fault does not fall within a vma or when
the target vma's flags don't support the fault's access permissions.
In those cases, continuing to try to resolve the fault won't cause
problems per-se, but it's wasteful and a bit confusing. Having
hva_to_pfn detect whether or not it may be possible to resolve the
fault asynchronously and return KVM_PFN_ERR_NEEDS_IO if so seems like
a good idea. It also matches what the existing code does.

-David