linux-kernel - Re: [PATCH] arm64/mm: Explicit cast conversions to correct data type

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Message-ID: <795a1d11-b47f-4ea1-aeab-484a434ea905@arm.com>
Date: Wed, 26 Feb 2025 12:19:37 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Mark Rutland <mark.rutland@....com>
Cc: Anshuman Khandual <anshuman.khandual@....com>,
 linux-arm-kernel@...ts.infradead.org,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64/mm: Explicit cast conversions to correct data type

On 25/02/2025 13:52, Mark Rutland wrote:
> On Tue, Feb 25, 2025 at 01:00:40PM +0000, Ryan Roberts wrote:
>> On 25/02/2025 12:32, Mark Rutland wrote:
>>> On Wed, Feb 19, 2025 at 09:26:46AM +0530, Anshuman Khandual wrote:
>>>> From: Ryan Roberts <ryan.roberts@....com>
>>>>
>>>> When CONFIG_ARM64_PA_BITS_52 is enabled, page table helpers __pte_to_phys()
>>>> and __phys_to_pte_val() are functions which return phys_addr_t and pteval_t
>>>> respectively as expected. But otherwise without this config being enabled,
>>>> they are defined as macros and their return types are implicit.
>>>>
>>>> Until now this has worked out correctly as both pte_t and phys_addr_t data
>>>> types have been 64 bits. But with the introduction of 128 bit page tables,
>>>> pte_t becomes 128 bits. Hence this ends up with incorrect widths after the
>>>> conversions, which leads to compiler warnings.
>>>
>>> Does 128-bit page table not imply 52-bit PAs?
>>
>> Not to my knowledge. For now the prototype code base is explicitly sticking to
>> 48-bit PA and 44-bit VA (for initial simplicitly because that's the limit for 4
>> levels).
> 
> Fair enough; info dump below, but hopefully nothing of consequence.
> 
> I assume that you're relying on the VMSAv9-128 PA bits [48:12] being in the
> same place as in the VMSAv8-64 descriptors, and being handled by the same
> PTE_ADDR_LOW mask that we use for CONFIG_ARM64_PA_BITS_52=n.

Yes that's what the prototype is doing for now.

> 
> From a quick scan of ARM DDI 0487 L.a, the VMSAv9-128 translation table
> descriptor format always contains a 56-bit PA (though PARange could be
> smaller than that). Bits [51:49] are packed differently than in
> VMSAv8-64 descriptors, and bits [55:52] are obviously new.

Indeed. But given we are running on a platform with 48 bit PA, the prototype
always treating [55:52] as 0 is not getting in the way of anything.

> 
>>>> Fix the warnings by explicitly casting to the correct type after doing the
>>>> conversion.
>>>
>>> I think it would be simpler and clearer if we replaced the macros with
>>> functions, such that __pte_to_phys() and __phys_to_pte_val() are
>>> *always* functions.
>>
>> Yeah, agreed. This was initially just a hack I did to get things working.
> 
> Cool; sounds like we're aligned.
> 
> Mark.