linux-kernel - Re: [PATCH 3/3] arm64: cpufeature: workaround AmpereOne FEAT

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Message-ID: <1a9c94ff-fad5-3440-705b-d8dc0c0b39b5@gentwo.org>
Date: Mon, 18 Nov 2024 10:33:01 -0800 (PST)
From: "Christoph Lameter (Ampere)" <cl@...two.org>
To: Yang Shi <yang@...amperecomputing.com>
cc: catalin.marinas@....com, will@...nel.org, scott@...amperecomputing.com, 
    linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] arm64: cpufeature: workaround AmpereOne FEAT_BBM
 level 2

On Mon, 18 Nov 2024, Yang Shi wrote:

> FEAT_BBM level 2 is not advertised on AmpereOne because of a bug when
> collapsing stage 2 mappings from smaller to larger translations.  That
> doesn't impact splitting stage 1 mappings (whether stage 2 is enabled or
> not), so workaround it by detecting CPUID.

Would be better to have a bblmv2_split_available() function that only
checks for the splitting capability.

If more code is added that uses the so far unused collapsing features
also included in the BBML2 feature set then that will break on AmpereOne.

bbml2_split_available() could call bbml2_available() and check the ampere
errata when false.

Should work fine for now.

Reviewed-by: Christoph Lameter <cl@...ux.com>