linux-kernel - Re: [PATCH] arm64/mm: Drop ESR_ELx_FSC

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Date: Thu, 13 Jun 2024 12:23:46 +0100
From: Marc Zyngier <maz@...nel.org>
To: Anshuman Khandual <anshuman.khandual@....com>
Cc: linux-arm-kernel@...ts.infradead.org,
	mark.rutland@....com,
	ryan.roberts@....com,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64/mm: Drop ESR_ELx_FSC_TYPE

On Thu, 13 Jun 2024 10:45:38 +0100,
Anshuman Khandual <anshuman.khandual@....com> wrote:
> 
> Fault status codes at page table level 0, 1, 2 and 3 for access, permission
> and translation faults are architecturally organized in a way, that masking
> out ESR_ELx_FSC_TYPE, fetches Level 0 status code for the respective fault.
> 
> Helpers like esr_fsc_is_[translation|permission|access_flag]_fault() mask
> out ESR_ELx_FSC_TYPE before comparing against corresponding Level 0 status
> code as the kernel does not yet care about the page table level, the fault
> really occurred previously.
> 
> This scheme is starting to crumble after FEAT_LPA2 when level -1 got added.
> Fault status code for translation fault at level -1 is 0x2B which does not
> follow ESR_ELx_FSC_TYPE, requiring esr_fsc_is_translation_fault() changes.
> 
> This changes above helpers to compare against individual fault status code
> values for each page table level and drop ESR_ELx_FSC_TYPE which is losing
> its value as a common mask.

I'd rather we do not drop the existing #defines, for a very
self-serving reason:

NV requires an implementation to synthesise fault syndromes, and these
definition are extensively used to compose the syndrome information
(see the NV MMU series at [1]). This is also heavily use to emulate
the AT instructions (fault reporting in PAR_EL1.FST).

Having additional helpers is fine. Dropping the base definitions
isn't, and I'd like to avoid reintroducing them.

Thanks,

	M.

[1] http://lore.kernel.org/r/20240529145628.3272630-1-maz@kernel.org

-- 
Without deviation from the norm, progress is not possible.