linux-kernel - Re: [PATCH RFC WIP] x86/paravirt: add register-saving thunks to reduce caller register pressure

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Message-Id: <1233183903.16317.242.camel@bodhitayantram.eng.vmware.com>
Date:	Wed, 28 Jan 2009 15:05:03 -0800
From:	Zachary Amsden <zach@...are.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC WIP] x86/paravirt: add register-saving thunks to
	reduce caller register pressure

On Wed, 2009-01-28 at 10:14 -0800, Jeremy Fitzhardinge wrote:
> Hi Guys,

> Zach: I wasn't really sure of what the VMI ABI calling convention is, so
> I haven't converted VMI yet.  I wasn't sure if it needs a register
> saving thunk to wrap the calls, or if they're guaranteed to not trash
> anything other than eax/rax.  Please advise.

C-like, allowing C or assembly implementations of calls.  Currently
several of the most important calls have a restricted clobber set,
however, and since it's clear there aren't any forthcoming new
implementations of a VMI ROM, it might be better to go with the actual
clobbers for performance critical ops.

> Standard calling convention is:
>          arguments          return      scratch
> x86-32   eax edx ecx        eax         ?

ecx, edx

> The most common paravirt ops, both statically and dynamically, are
> interrupt enable/disable/save/restore, so handle them first.  This is
> particularly easy since their calls are handled specially anyway.

> XXX Deal with VMI.  What's their calling convention?

Enable/Disable have no clobbers at all.
Save clobbers only return value, %eax
Restore also clobbers nothing.

The patching code has gotten quite complex with the 32/64 union; let me
apply it first and see before I comment on the patch.

Zach

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