linux-kernel - Re: [PATCH v1 01/11] arm/pgtable: define PFN_PTE

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Message-ID: <7f39e2db-0fa1-4391-956f-d2586fa234bd@arm.com>
Date: Tue, 23 Jan 2024 11:44:35 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, Russell King <linux@...linux.org.uk>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Dinh Nguyen <dinguyen@...nel.org>, Michael Ellerman <mpe@...erman.id.au>,
 Nicholas Piggin <npiggin@...il.com>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
 "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt
 <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Alexander Gordeev <agordeev@...ux.ibm.com>,
 Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
 Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>, "David S. Miller"
 <davem@...emloft.net>, linux-arm-kernel@...ts.infradead.org,
 linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
 linux-s390@...r.kernel.org, sparclinux@...r.kernel.org
Subject: Re: [PATCH v1 01/11] arm/pgtable: define PFN_PTE_SHIFT on arm and
 arm64

On 23/01/2024 11:33, David Hildenbrand wrote:
> On 23.01.24 12:17, Ryan Roberts wrote:
>> On 23/01/2024 11:02, David Hildenbrand wrote:
>>> 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@...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));
>>>>> }
>>>>>
>>>
>>> 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?
>>>
>>
>>
>> But __pte_to_phys() and __phys_to_pte_val() depend on a Kconfig. If PA bits is
>> 52, the bits are not all contiguous:
>>
>> #ifdef CONFIG_ARM64_PA_BITS_52
>> static inline phys_addr_t __pte_to_phys(pte_t pte)
>> {
>>     return (pte_val(pte) & PTE_ADDR_LOW) |
>>         ((pte_val(pte) & PTE_ADDR_HIGH) << PTE_ADDR_HIGH_SHIFT);
>> }
>> static inline pteval_t __phys_to_pte_val(phys_addr_t phys)
>> {
>>     return (phys | (phys >> PTE_ADDR_HIGH_SHIFT)) & PTE_ADDR_MASK;
>> }
>> #else
>> #define __pte_to_phys(pte)    (pte_val(pte) & PTE_ADDR_MASK)
>> #define __phys_to_pte_val(phys)    (phys)
>> #endif
>>
> 
> Ah, how could I've missed that. Agreed, set_ptes() and this patch are broken.
> 
> Do you want to send a patch to implement pte_next_pfn() on arm64, and then use
> pte_next_pfn() in set_ptes()? Then I can drop this patch here completely from
> this series.

Yes good idea. I probably won't get around to it until tomorrow.