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

From: Yu Zhang <[email-masked]>
Date: Thu, 6 Jul 2023 22:52:47 +0800

On Thu, Jul 06, 2023 at 02:29:24PM +0900, David Stevens wrote:
> On Wed, Jul 5, 2023 at 7:53 PM Yu Zhang <[email-masked]> wrote:
> >
> > 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?
> 
> We just set the foll->writable out parameter to be equal to
> ((foll->flags & FOLL_WRITE) && foll->allow_write_mapping). Taking a =
> foll->flags & FOLL_WRITE and b = foll->allow_write_mapping, we have
> !(a && b) && b -> (!a || !b) && b -> (!a && b) || (!b && b) -> !a &&
> b.

Ouch, my bad again... I typed "!foll->writable", but missed the "!" in
my head while calculating... Thanks! :)

B.R.
Yu