linux-kernel - Re: [PATCH v5 08/15] x86/mtrr: have only one set

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Date:   Wed, 12 Apr 2023 14:56:33 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v5 08/15] x86/mtrr: have only one set_mtrr() variant

On 12.04.23 14:30, Borislav Petkov wrote:
> On Sat, Apr 01, 2023 at 08:36:45AM +0200, Juergen Gross wrote:
>> Today there are two variants of set_mtrr(): one calling stop_machine()
> 
> "... two variants which set MTRRs: set_mtrr() and set_mtrr_cpuslocked(),
> first calling ..."
> 
>> and one calling stop_machine_cpuslocked().
>>
>> The first one (set_mtrr()) has only one caller, and this caller is
>> always running with only one CPU online and interrupts being off.
> 
> Wait, whaaat?
> 
> It's only caller is mtrr_restore() and that is part of syscorse ops
> which is registered for
> 
> "The CPU has no MTRR and seems to not support SMP."
> 
> Do you mean that, per chance?

I'm not sure the comment is accurate ("has no MTRR" is a bad way
to say that X86_FEATURE_MTRR is 0, BTW).

The main point is that mtrr_restore() is called with interrupts off
and before any other potential CPUs are started again.

> If you do, please explain that properly in the commit message - this is
> not a guessing game.

Okay.

> By the looks of that syscore thing, it is needed for the very old MTRR
> implementations which weren't SMP (K6, Centaur, Cyrix etc).
> 
> Please explain that in the commit message too. It needs to say *why* the
> transformation you're doing is ok.

I've outlined the "why" above. I'll replace the paragraph you were referring
to.

> 
> "this caller is always running with only one CPU online" is not nearly
> beginning to explain what the situation is.
> 
>> Remove the first variant completely and replace the call of it with
>> a call of mtrr_if->set().
>>
>> Rename the second variant set_mtrr_cpuslocked() to set_mtrr() now that
>> there is only one variant left.
> 
> Those are visible from the diff itself - you don't need to explain what
> the patch does but why.

Okay, I'll drop that paragraph.


Juergen

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