linux-kernel - Re: [Xen-devel] Re: [PATCH] xen: use iret directly where possible

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Date:	Mon, 4 Jun 2007 23:46:54 +0200
From:	Andi Kleen <ak@...e.de>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Virtualization Mailing List <virtualization@...ts.osdl.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] Re: [PATCH] xen: use iret directly where possible

On Monday 04 June 2007 23:28, Jeremy Fitzhardinge wrote:

> The cli/sti instructions don't control the event mask, so they're
> effectively expensive no-ops (they trap into the hypervisor, are
> emulated as no-ops). But if you mean cli as a general term for
> "events/interrupts masked", then they can't remain masked when you
> return to usermode.   iret normally sets the current eflags IF state
> from the on-stack IF, but that's irrelevent to Xen; we need to extract
> eflags.IF from the on-stack eflags, and put that into the vcpu's event
> mask.  That's inherently non-atomic with respect to iret.

Ah I assumed the hypervisor would just check IF in ring 1 too.
It would certainly make this easier, but then the additional trap
of setting it would be also somewhat expensive agreed.

I must say I still hate the patch; it has all the signs of something that
will be very nasty to maintain later.

-Andi
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