linux-kernel - Re: [patch 3/3] x86/paravirt: Replace paravirt patch asm magic

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Message-ID: <20190425092049.GB115378@gmail.com>
Date:   Thu, 25 Apr 2019 11:20:49 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Juergen Gross <jgross@...e.com>,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [patch 3/3] x86/paravirt: Replace paravirt patch asm magic


* Peter Zijlstra <peterz@...radead.org> wrote:

> > I think these open-coded hexa versions are somewhat fragile as well, how 
> > about putting these into a .S file and controlling the sections in an LTO 
> > safe manner there?
> > 
> > That will also allow us to write proper asm, and global labels can be 
> > used to extract the patchlets and their length?
> 
> While I'm not fan either; I think that will be worse still, because it
> splits the information over multiple files.

Yeah, so that's a drawback of the .S files.

> The advantage of this form is that it is clear how long the instructions
> are, which is important for the patching. These immediates have to be
> shorter than 5 bytes because they overwrite the CALL/JMP to the paravirt
> function.
> 
> /me eyes .cpu_usergs_sysret64 and goes wtf..

I just posted a patch that adds an assert to detect too large patching 
attempt: we'd silently ignore them before, which isn't healthy.

With the two patches I now like the .c version better.

Thomas, want me to organize all these changes, or do you want to?
1
Thanks,

	Ingo