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

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Message-ID: <Z73LEkHklpjvIlmZ@J2N7QTR9R3.cambridge.arm.com>
Date: Tue, 25 Feb 2025 13:52:18 +0000
From: Mark Rutland <mark.rutland@....com>
To: Ryan Roberts <ryan.roberts@....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 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.

>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.

> >> 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.