linux-kernel - Re: [PATCH v2 2/3] x86/smp: Allow forcing the mwait hint for play dead loop

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Message-ID: <7c08ebf7-69dd-44c1-b43e-fb9b50704dd6@intel.com>
Date: Wed, 6 Nov 2024 06:46:18 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>,
 Patryk Wlazlyn <patryk.wlazlyn@...ux.intel.com>, x86@...nel.org
Cc: linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
 rafael.j.wysocki@...el.com, len.brown@...el.com, dave.hansen@...ux.intel.com
Subject: Re: [PATCH v2 2/3] x86/smp: Allow forcing the mwait hint for play
 dead loop

On 11/6/24 00:14, Artem Bityutskiy wrote:
> The non-architectural part of it is basically this.
> 
> The algorithm assumes that in the sub-state part of the mwait hint (4 least
> significant bits) always starts with 0 and then next sub-state is always 1, and
> there are no gaps. This is not documented in Intel SDM. This happens to work on
> majority of Intel platforms, but this is not architectural.

I wouldn't call that a "non-architectural algorithm".  It's a bug, plain
and simple.