From mboxrd@z Thu Jan  1 00:00:00 1970
From: "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH] Add I/O hypercalls for i386 paravirt
Date: Tue, 21 Aug 2007 23:00:21 -0700
Message-ID: <46CBD0F5.2080709@zytor.com>
References: <46CBC842.4070100@vmware.com>
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Sender: linux-kernel-owner@vger.kernel.org
To: Zachary Amsden <zach@vmware.com>
Cc: Andrew Morton <akpm@osdl.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Virtualization Mailing List <virtualization@lists.osdl.org>, Rusty Russell <rusty@rustcorp.com.au>, Chris Wright <chrisw@sous-sol.org>, Avi Kivity <avi@qumranet.com>, Jeremy Fitzhardinge <jeremy@goop.org>
List-Id: virtualization@lists.linuxfoundation.org

Zachary Amsden wrote:
> In general, I/O in a virtual guest is subject to performance problems. 
> The I/O can not be completed physically, but must be virtualized.  This
> means trapping and decoding port I/O instructions from the guest OS. 
> Not only is the trap for a #GP heavyweight, both in the processor and
> the hypervisor (which usually has a complex #GP path), but this forces
> the hypervisor to decode the individual instruction which has faulted. 
> Worse, even with hardware assist such as VT, the exit reason alone is
> not sufficient to determine the true nature of the faulting instruction,
> requiring a complex and costly instruction decode and simulation.
> 
> This patch provides hypercalls for the i386 port I/O instructions, which
> vastly helps guests which use native-style drivers.  For certain VMI
> workloads, this provides a performance boost of up to 30%.  We expect
> KVM and lguest to be able to achieve similar gains on I/O intensive
> workloads.
> 

What about cost on hardware?

	-hpa