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

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Date:	Wed, 22 Aug 2007 12:22:17 +0200
From:	Andi Kleen <ak@...e.de>
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

On Tue, Aug 21, 2007 at 10:23:14PM -0700, 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.  

Is that really that expensive? Hard to imagine.

e.g. you could always have a fast check for inb/outb at the beginning
of the #GP handler. And is your initial #GP entry really more expensive
than a hypercall? 

> 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.

It's unclear to me why that should be that costly.

Worst case it's a switch()

-Andi
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