Re: [PATCH v1 01/11] arm/pgtable: define PFN_PTE_SHIFT on arm and arm64

Ryan Roberts
2024-01-23T11:08:43Z

On 23/01/2024 10:48, David Hildenbrand wrote:
> On 23.01.24 11:34, Ryan Roberts wrote:
>> On 22/01/2024 19:41, David Hildenbrand wrote:
>>> We want to make use of pte_next_pfn() outside of set_ptes(). Let's
>>> simpliy define PFN_PTE_SHIFT, required by pte_next_pfn().
>>>
>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>> ---
>>>   arch/arm/include/asm/pgtable.h   | 2 ++
>>>   arch/arm64/include/asm/pgtable.h | 2 ++
>>>   2 files changed, 4 insertions(+)
>>>
>>> diff --git a/arch/arm/include/asm/pgtable.h b/arch/arm/include/asm/pgtable.h
>>> index d657b84b6bf70..be91e376df79e 100644
>>> --- a/arch/arm/include/asm/pgtable.h
>>> +++ b/arch/arm/include/asm/pgtable.h
>>> @@ -209,6 +209,8 @@ static inline void __sync_icache_dcache(pte_t pteval)
>>>   extern void __sync_icache_dcache(pte_t pteval);
>>>   #endif
>>>   +#define PFN_PTE_SHIFT        PAGE_SHIFT
>>> +
>>>   void set_ptes(struct mm_struct *mm, unsigned long addr,
>>>                 pte_t *ptep, pte_t pteval, unsigned int nr);
>>>   #define set_ptes set_ptes
>>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>>> index 79ce70fbb751c..d4b3bd96e3304 100644
>>> --- a/arch/arm64/include/asm/pgtable.h
>>> +++ b/arch/arm64/include/asm/pgtable.h
>>> @@ -341,6 +341,8 @@ static inline void __sync_cache_and_tags(pte_t pte,
>>> unsigned int nr_pages)
>>>           mte_sync_tags(pte, nr_pages);
>>>   }
>>>   +#define PFN_PTE_SHIFT        PAGE_SHIFT
>>
>> I think this is buggy. And so is the arm64 implementation of set_ptes(). It
>> works fine for 48-bit output address, but for 52-bit OAs, the high bits are not
>> kept contigously, so if you happen to be setting a mapping for which the
>> physical memory block straddles bit 48, this won't work.
>
> Right, as soon as the PTE bits are not contiguous, this stops working, just like
> set_ptes() would, which I used as orientation.
>
>>
>> Today, only the 64K base page config can support 52 bits, and for this,
>> OA[51:48] are stored in PTE[15:12]. But 52 bits for 4K and 16K base pages is
>> coming (hopefully v6.9) and in this case OA[51:50] are stored in PTE[9:8].
>> Fortunately we already have helpers in arm64 to abstract this.
>>
>> So I think arm64 will want to define its own pte_next_pfn():
>>
>> #define pte_next_pfn pte_next_pfn
>> static inline pte_t pte_next_pfn(pte_t pte)
>> {
>>     return pfn_pte(pte_pfn(pte) + 1, pte_pgprot(pte));
>> }
>>
>> I'll do a separate patch to fix the already broken arm64 set_ptes()
>> implementation.
>
> Make sense.
>
>>
>> I'm not sure if this type of problem might also apply to other arches?
>
> I saw similar handling in the PPC implementation of set_ptes, but was not able
> to convince me that it is actually required there.
>
> pte_pfn on ppc does:
>
> static inline unsigned long pte_pfn(pte_t pte)
> {
>     return (pte_val(pte) & PTE_RPN_MASK) >> PTE_RPN_SHIFT;
> }
>
> But that means that the PFNs *are* contiguous.

all the ppc pfn_pte() implementations also only shift the pfn, so I think ppc is
safe to just define PFN_PTE_SHIFT. Although 2 of the 3 implementations shift by
PTE_RPN_SHIFT and the other shifts by PAGE_SIZE, so you might want to define
PFN_PTE_SHIFT separately for all 3 configs?

> If high bits are used for
> something else, then we might produce a garbage PTE on overflow, but that
> shouldn't really matter I concluded for folio_pte_batch() purposes, we'd not
> detect "belongs to this folio batch" either way.

Exactly.

>
> Maybe it's likely cleaner to also have a custom pte_next_pfn() on ppc, I just
> hope that we don't lose any other arbitrary PTE bits by doing the pte_pgprot().

I don't see the need for ppc to implement pte_next_pfn().

pte_pgprot() is not a "proper" arch interface (its only required by the core-mm
if the arch implements a certain Kconfig IIRC). For arm64, all bits that are not
pfn are pgprot, so there are no bits lost.

>
>
> I guess pte_pfn() implementations should tell us if anything special needs to
> happen.
>