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Date:   Tue, 15 Mar 2022 18:12:16 +0100
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Subject: Re: [PATCH v1 5/7] s390/pgtable: support
 __HAVE_ARCH_PTE_SWP_EXCLUSIVE
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From:   David Hildenbrand <david@redhat.com>
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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.

-- 
Thanks,

David / dhildenb