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

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Date:	Tue, 21 Aug 2007 23:00:21 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Zachary Amsden <zach@...are.com>
CC:	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

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
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