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

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Message-ID: <46CBCADF.2070400@qumranet.com>
Date:	Wed, 22 Aug 2007 08:34:23 +0300
From:	Avi Kivity <avi@...ranet.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>,
	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.
>


Won't these workloads be better off using paravirtualized drivers? 
i.e., do the native drivers with paravirt I/O instructions get anywhere
near the performance of paravirt drivers?


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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