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Date:   Wed, 16 Mar 2022 11:56:54 +0100
From:   Gerald Schaefer <gerald.schaefer@linux.ibm.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        Shakeel Butt <shakeelb@google.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Yang Shi <shy828301@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>, Jann Horn <jannh@google.com>,
        Michal Hocko <mhocko@kernel.org>,
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        Heiko Carstens <hca@linux.ibm.com>,
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        Christian Borntraeger <borntraeger@linux.ibm.com>
Subject: Re: [PATCH v1 5/7] s390/pgtable: support
 __HAVE_ARCH_PTE_SWP_EXCLUSIVE
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On Tue, 15 Mar 2022 18:12:16 +0100
David Hildenbrand <david@redhat.com> 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.