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Date: Tue, 23 Jan 2024 12:02:20 +0100
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Subject: Re: [PATCH v1 01/11] arm/pgtable: define PFN_PTE_SHIFT on arm and
 arm64
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From: David Hildenbrand <david@redhat.com>
To: Ryan Roberts <ryan.roberts@arm.com>, linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
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Organization: Red Hat
In-Reply-To: <46080ac1-7789-499b-b7f3-0231d7bd6de7@redhat.com>
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On 23.01.24 11:48, 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));
>> }
>>

Digging into the details, on arm64 we have:

#define pte_pfn(pte)           (__pte_to_phys(pte) >> PAGE_SHIFT)

and

#define __pte_to_phys(pte)     (pte_val(pte) & PTE_ADDR_MASK)

But that implies, that upstream the PFN is always contiguous, no?

-- 
Cheers,

David / dhildenb