linux-kernel - Re: [PATCH 0/3] x86: make pat and mtrr independent from each other

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Message-ID: <efbde93b-e280-0e40-798d-dc7bf8ca83cf@leemhuis.info>
Date:   Sun, 17 Jul 2022 09:55:00 +0200
From:   Thorsten Leemhuis <regressions@...mhuis.info>
To:     Juergen Gross <jgross@...e.com>, xen-devel@...ts.xenproject.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org
Cc:     brchuckz@...scape.net, jbeulich@...e.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "# 5 . 17" <stable@...r.kernel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Pavel Machek <pavel@....cz>, Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [PATCH 0/3] x86: make pat and mtrr independent from each other

Hi Juergen!

On 15.07.22 16:25, Juergen Gross wrote:
> Today PAT can't be used without MTRR being available, unless MTRR is at
> least configured via CONFIG_MTRR and the system is running as Xen PV
> guest. In this case PAT is automatically available via the hypervisor,
> but the PAT MSR can't be modified by the kernel and MTRR is disabled.
> 
> As an additional complexity the availability of PAT can't be queried
> via pat_enabled() in the Xen PV case, as the lack of MTRR will set PAT
> to be disabled. This leads to some drivers believing that not all cache
> modes are available, resulting in failures or degraded functionality.
> 
> The same applies to a kernel built with no MTRR support: it won't
> allow to use the PAT MSR, even if there is no technical reason for
> that, other than setting up PAT on all cpus the same way (which is a
> requirement of the processor's cache management) is relying on some
> MTRR specific code.
> 
> Fix all of that by:
> 
> - moving the function needed by PAT from MTRR specific code one level
>   up
> - adding a PAT indirection layer supporting the 3 cases "no or disabled
>   PAT", "PAT under kernel control", and "PAT under Xen control"
> - removing the dependency of PAT on MTRR

Thx for working on this. If you need to respin these patches for one
reason or another, could you do me a favor and add proper 'Link:' tags
pointing to all reports about this issue? e.g. like this:

 Link: https://lore.kernel.org/regressions/YnHK1Z3o99eMXsVK@mail-itl/

These tags are considered important by Linus[1] and others, as they
allow anyone to look into the backstory weeks or years from now. That is
why they should be placed in cases like this, as
Documentation/process/submitting-patches.rst and
Documentation/process/5.Posting.rst explain in more detail. I care
personally, because these tags make my regression tracking efforts a
whole lot easier, as they allow my tracking bot 'regzbot' to
automatically connect reports with patches posted or committed to fix
tracked regressions.

[1] see for example:
https://lore.kernel.org/all/CAHk-=wjMmSZzMJ3Xnskdg4+GGz=5p5p+GSYyFBTh0f-DgvdBWg@mail.gmail.com/
https://lore.kernel.org/all/CAHk-=wgs38ZrfPvy=nOwVkVzjpM3VFU1zobP37Fwd_h9iAD5JQ@mail.gmail.com/
https://lore.kernel.org/all/CAHk-=wjxzafG-=J8oT30s7upn4RhBs6TX-uVFZ5rME+L5_DoJA@mail.gmail.com/

Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)

P.S.: As the Linux kernel's regression tracker I deal with a lot of
reports and sometimes miss something important when writing mails like
this. If that's the case here, don't hesitate to tell me in a public
reply, it's in everyone's interest to set the public record straight.

BTW, let me tell regzbot to monitor this thread:

#regzbot ^backmonitor:
https://lore.kernel.org/regressions/YnHK1Z3o99eMXsVK@mail-itl/