Date: Tue, 23 Jan 2024 11:16:53 +0000
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Ryan Roberts <ryan.roberts@....com>, David Hildenbrand <david@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
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Subject: Re: [PATCH v1 01/11] arm/pgtable: define PFN_PTE_SHIFT on arm and
 arm64



Le 23/01/2024 à 12:08, Ryan Roberts a écrit :
> 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@...hat.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?

We have PTE_RPN_SHIFT defined for all 4 implementations, for some of 
them you are right it is defined as PAGE_SHIFT, but I see no reason to 
define PFN_PTE_SHIFT separately.

> 
>> 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().

Agreed.

> 
> 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.
>>
> 

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