Re: [RFC PATCH] arm64: Cache HW update of Access Flag support

From: Gabriel Krisman Bertazi <krisman@suse.de>
To: Will Deacon <will@kernel.org>
Cc: catalin.marinas@arm.com,  linux-arm-kernel@lists.infradead.org,
	broonie@kernel.org
Subject: Re: [RFC PATCH] arm64: Cache HW update of Access Flag support
Date: Mon, 09 Jan 2023 12:16:13 -0300	[thread overview]
Message-ID: <87lembg2ma.fsf@suse.de> (raw)
In-Reply-To: <20230106170949.GA5019@willie-the-truck> (Will Deacon's message of "Fri, 6 Jan 2023 17:09:50 +0000")

Will Deacon <will@kernel.org> writes:

> On Fri, Jan 06, 2023 at 01:38:25PM -0300, Gabriel Krisman Bertazi wrote:
>> +		has_af = cpuid_feature_extract_unsigned_field(
>> +						read_cpuid(ID_AA64MMFR1_EL1),
>>  						ID_AA64MMFR1_EL1_HAFDBS_SHIFT);
>> +	return has_af;
>
> The intention here was to read the value for the _current_ CPU, since it
> might not be the same across the system when big.MISTAKE gets involved.
>
> However, since this is really just a performance optimisation and I
> think that the access flag tends to be uniformly supported in practice
> anyway, the best bet is probably just to read the sanitised version of
> the field using read_sanitised_ftr_reg().
>
> Can you give that a shot, please?

Hey Will,

Thanks for the review.

I reran the benchmark over the weekend and the impact of the bsearch
lookup seems to be <1% worse than my original patch, which is
negligible, IMO.  I will shortly follow up with a v2 applying your
suggestion.

Thanks!

-- 
Gabriel Krisman Bertazi

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