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

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Message-ID: <ca66f7ac-97d5-474d-bec4-d0ff79c08eaa@arm.com>
Date: Fri, 14 Jun 2024 07:54:53 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Marc Zyngier <maz@...nel.org>
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 6/13/24 16:53, Marc Zyngier wrote:
> 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.

You would like to just leave behind all the existing level 0 syndrome macro
definitions in place ?

#define ESR_ELx_FSC_ACCESS	(0x08)
#define ESR_ELx_FSC_FAULT	(0x04)
#define ESR_ELx_FSC_PERM	(0x0C)

Or which are rather

#define ESR_ELx_FSC_ACCESS	ESR_ELx_FSC_ACCESS_L0
#define ESR_ELx_FSC_FAULT	ESR_ELx_FSC_FAULT_L0
#define ESR_ELx_FSC_PERM	ESR_ELx_FSC_PERM_L0

But just wondering why cannot ESR_ELx_FSC_[ACCESS|FAULT|PERM]_L0 definitions
be used directly in new use cases ?

> 
> Thanks,
> 
> 	M.
> 
> [1] http://lore.kernel.org/r/20240529145628.3272630-1-maz@kernel.org
>