Re: [GIT PULL] x86/shstk for 6.4

[Linus Torvalds]

On Sat, May 6, 2023 at 5:18 PM Edgecombe, Rick P
<rick.p.edgecombe@xxxxxxxxx> wrote:
>
> On Sat, 2023-05-06 at 13:09 -0700, Linus Torvalds wrote:
> > >
> > >  (b) "pte_dirty()" isn't even the right thing to use, since it
> > > includes the SW dirty bit.
>
> pte_dirty() actually doesn't check the the SW dirty bit

It really does.

See commit "x86/mm: Start actually marking _PAGE_SAVED_DIRTY", where
the patch is:

  -static inline int pte_dirty(pte_t pte)
  +static inline bool pte_dirty(pte_t pte)
   {
  -     return pte_flags(pte) & _PAGE_DIRTY;
  +     return pte_flags(pte) & _PAGE_DIRTY_BITS;
   }

and that  _PAGE_DIRTY_BITS is a combination of _PAGE_DIRTY | _PAGE_SAVED_DIRTY.

So that means that the

        if (pte_dirty(pte))
                pte = pte_mksaveddirty(pte);

in pte_wrprotect() is just nonsensical, and basically says "if either
the real dirty or the SW dirty bit is set, set the SW dirty bit". But
that's entirely redundant wrt the old state of the dirty bit.

It reality should just 'or' the HW dirty bit into the SW dirty bit and
be done with it.

Of course, maybe I confused the issue by talking about HW dirty and SW
dirty, because we *also* have that entirely *other* legacy
"SOFT_DIRTY" bit that is different from the new SW dirty bit
("SAVED_DIRTY").

Which certainly doesn't make things clearer.  The "soft" in the soft
dirty bit isn't about software, (although it's obviously done *in*
software), it's literally "soft" not as in software, but as in just
"not a hard bit": you can clear it for tracking which pages people
write to.

So we end up with two bits that are maintained by software: SOFT and
SAVED, and then it's really easy to just call the "SAVED" bit the "SW
dirty" bit as I did as opposed to the "HW dirty" bit, because it
really is a secondary very of the *hard* HW dirty bit.

I have this fleeting suspicion that we could actually merge the SOFT
and SAVED bits, in that the "SOFT" bit doesn't make much sense if the
pte is read-only, and the SAVED bit in turn doesn't matter if the pte
is RW.

But that's a separate headache, not worth worrying about now.

           Linus