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

[David Hildenbrand <david@redhat.com>]

On 16.03.22 14:27, Gerald Schaefer wrote:
> On Wed, 16 Mar 2022 14:01:07 +0100
> Christian Borntraeger <borntraeger@linux.ibm.com> wrote:
> 
>>
>>
>> Am 16.03.22 um 11:56 schrieb Gerald Schaefer:
>>> 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.
>>
>> 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.
> 
> Ok, bit 55 would then also be an option IIUC, since execution protection
> should not be relevant for swap ptes. And Davids clean-up removing the
> restriction for bit 52 and 55 in the comment would make sense.
> 
> I would also favor bit 52 though (PAGE_LARGE), as in Davids initial patch
> version, since this is never used for any real ptes. The PAGE_LARGE flag
> is only set in the "virtual" large ptes that the hugetlb code is seeing
> from huge_ptep_get(). But it will (and must) never be written as a valid
> pte, or else it will generate an exception. IIRC, we only set it to detect
> such possible bugs, e.g. hugetlb code writing a pte (which really is a
> pmd/pud) directly, instead of using set_huge_pte_at().
> 

Agreed. I'll include the doc cleanup patch and a fixed-up version of
this patch (still using bit 52, not messing with the offset bits) in the
next version.

Thanks all!

-- 
Thanks,

David / dhildenb