linux-kernel - Re: [RFC PATCH 2/5] entry: Add calls for save/restore auxiliary pt

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Date:   Wed, 10 Aug 2022 01:53:49 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ira Weiny <ira.weiny@...el.com>, Borislav Petkov <bp@...en8.de>
Cc:     Rik van Riel <riel@...riel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [RFC PATCH 2/5] entry: Add calls for save/restore auxiliary
 pt_regs

On Tue, Aug 09 2022 at 14:49, Ira Weiny wrote:
> On Tue, Aug 09, 2022 at 08:49:47PM +0200, Borislav Petkov wrote:
>> Why does that need to run with instrumentation enabled?
>
> This does not.
>
> Am I wrong that instrumentation begin/end are 0 overhead in production builds?

instrumentation_begin/end() are annotations and never emit executable
code. Neither in production nor in debug builds. Why keep you harping on
this question? Is it so hard to figure out what it does?

But that's not the question at all. The question is where auxregs need
to be set up.

Once the entry code leaves non-instrumentable code it's obviously
desired to have consistent state for instrumentation.

This obviously applies also to the auxreg space unless there is a
compelling reason why this is not required.

Thanks,

        tglx