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

From: Yu Zhang <[email-masked]>
Date: Wed, 5 Jul 2023 18:53:43 +0800

On Wed, Jul 05, 2023 at 06:22:59PM +0900, David Stevens wrote:
> On Wed, Jul 5, 2023 at 12:10 PM Yu Zhang <[email-masked]> 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.

Well, as the comment says, we wanna try to map the read fault as writable
whenever possible. And __gfn_to_pfn_memslot() will only set the FOLL_WRITE
for write faults. So I guess using !foll->writable will not allow this.
Did I miss anything?

> > > +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.

Got it. Sounds reasonable. And thanks! :)

B.R.
Yu