linux-kernel - Re: [PATCH] drm/vmwgfx: Silence RBP clobber warnings

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Date:   Fri, 2 Jun 2023 08:32:39 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Zack Rusin <zackr@...are.com>, linux-kernel@...r.kernel.org,
        Miroslav Benes <mbenes@...e.cz>, x86@...nel.org,
        VMware Graphics Reviewers 
        <linux-graphics-maintainer@...are.com>,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH] drm/vmwgfx: Silence RBP clobber warnings

On Fri, Jun 02, 2023 at 05:16:39PM +0200, Peter Zijlstra wrote:
> On Fri, Jun 02, 2023 at 07:56:34AM -0700, Josh Poimboeuf wrote:
> > VMware hypercalls take the RBP register as input.  This breaks basic
> > frame pointer convention, as RBP should never be clobbered.
> > 
> > So frame pointer unwinding is broken for the instructions surrounding
> > the hypercall with the clobbered RBP.  There's nothing that can be done
> > about that.  Just tell objtool to ignore it.
> > 
> 
> That's a pretty horrific ABI, one that violates the oldest x86 calling
> convention in existence.
> 
> VMware folks, shame!!

Agreed :-(

BTW, please ignore the patch, I'll be sending a v2.

I realized (with Peter's prodding on IRC) that reliable ORC unwinding
would be broken if the function has a frame pointer.  Which can
happen if the function needs an aligned stack.

-- 
Josh