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

[Zachary Amsden <zach@vmware.com>]

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