linux-kernel - Re: [PATCH] Add I/O hypercalls for i386 paravirt

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Date:	Wed, 22 Aug 2007 18:34:31 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Andi Kleen <ak@...e.de>
Cc:	Zachary Amsden <zach@...are.com>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...l.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Virtualization Mailing List <virtualization@...ts.osdl.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Chris Wright <chrisw@...s-sol.org>,
	Avi Kivity <avi@...ranet.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: [PATCH] Add I/O hypercalls for i386 paravirt

> out is usually a single byte. Shouldn't be very expensive
> to decode. In fact it should be roughly equivalent to your
> hypercall multiplex.

Why is a performance critical path on a paravirt kernel even using I/O
instructions and not paravirtual device drivers ?

It clearly makes sense to virtualise I/O operations if you are doing that
(so you can do posting, triggers and predicted reply handling guest side
to keep the trap rate sane) but I don't see why this situation occurs in
the first place for paravirt.

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