Re: [PATCH v1 5/7] s390/pgtable: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE

[Christian Borntraeger]

Am 16.03.22 um 11:56 schrieb Gerald Schaefer:
> On Tue, 15 Mar 2022 18:12:16 +0100
> David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> > On 15.03.22 17:58, David Hildenbrand wrote:
> > > > > This would mean that it is not OK to have bit 52 not zero for swap PTEs.
> > > > > But if I read the POP correctly, all bits except for the DAT-protection
> > > > > would be ignored for invalid PTEs, so maybe this comment needs some update
> > > > > (for both bits 52 and also 55).
> > > > >
> > > > > Heiko might also have some more insight.
> > > >
> > > > Indeed, I wonder why we should get a specification exception when the
> > > > PTE is invalid. I'll dig a bit into the PoP.
> > >
> > > SA22-7832-12 6-46 ("Translation-Specification Exception") is clearer
> > >
> > > "The page-table entry used for the translation is
> > > valid, and bit position 52 does not contain zero."
> > >
> > > "The page-table entry used for the translation is
> > > valid, EDAT-1 does not apply, the instruction-exe-
> > > cution-protection facility is not installed, and bit
> > > position 55 does not contain zero. It is model
> > > dependent whether this condition is recognized."
> >
> > I wonder if the following matches reality:
> >
> > diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h
> > index 008a6c856fa4..6a227a8c3712 100644
> > --- a/arch/s390/include/asm/pgtable.h
> > +++ b/arch/s390/include/asm/pgtable.h
> > @@ -1669,18 +1669,16 @@ static inline int has_transparent_hugepage(void)
> >   /*
> >    * 64 bit swap entry format:
> >    * A page-table entry has some bits we have to treat in a special way.
> > - * Bits 52 and bit 55 have to be zero, otherwise a specification
> > - * exception will occur instead of a page translation exception. The
> > - * specification exception has the bad habit not to store necessary
> > - * information in the lowcore.
> >    * Bits 54 and 63 are used to indicate the page type.
> >    * A swap pte is indicated by bit pattern (pte & 0x201) == 0x200
> > - * This leaves the bits 0-51 and bits 56-62 to store type and offset.
> > - * We use the 5 bits from 57-61 for the type and the 52 bits from 0-51
> > - * for the offset.
> > - * |                     offset                        |01100|type |00|
> > + * |                     offset                        |XX1XX|type |S0|
> >    * |0000000000111111111122222222223333333333444444444455|55555|55566|66|
> >    * |0123456789012345678901234567890123456789012345678901|23456|78901|23|
> > + *
> > + * Bits 0-51 store the offset.
> > + * Bits 57-62 store the type.
> > + * Bit 62 (S) is used for softdirty tracking.
> > + * Bits 52, 53, 55 and 56 (X) are unused.
> >    */
> >   
> >   #define __SWP_OFFSET_MASK      ((1UL << 52) - 1)
> >
> >
> > I'm not sure why bit 53 was indicated as "1" and bit 55 was indicated as
> > "0". At least for 52 and 55 there was a clear description.
>
> Bit 53 is the invalid bit, and that is always 1 for swap ptes, in addition
> to protection bit 54. Bit 55, along with bit 52, has to be zero according
> to the (potentially deprecated) comment.
>
> It is interesting that bit 56 seems to be unused, at least according
> to the comment, but that would also mention bit 62 as unused, so that
> clearly needs some update.
>
> If bit 56 could be used for _PAGE_SWP_EXCLUSIVE, that would be better
> than stealing a bit from the offset, or using potentially dangerous
> bit 52. It is defined as _PAGE_UNUSED and only used for kvm, not sure
> if this is also relevant for swap ptes, similar to bit 62.
>
> Adding Christian on cc, maybe he has some insight on _PAGE_UNUSED
> bit 56 and swap ptes.

I think _PAGE_UNUSED is not used for swap ptes. It is used _before_ swapping
to decide whether we swap or discard the page.

Regarding bit 52, the POP says in chapter 3 for the page table entry

[..]
Page-Invalid Bit (I): Bit 53 controls whether the
page associated with the page-table entry is avail-
able. When the bit is zero, address translation pro-
ceeds by using the page-table entry. When the bit is
one, the page-table entry cannot be used for transla-
tion.


-->When the page-invalid bit is one, all other bits in the
-->page-table entry are available for use by program-
-->ming.

this was added with the z14 POP, but I guess it was just a clarification
and should be valid for older machines as well.
So 52 and 56 should be ok, with 52 probably the better choice.

PS: the page protect bit is special and should not be used (bit54) for
KVM related reasons