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

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Date:	Tue, 21 Aug 2007 22:23:14 -0700
From:	Zachary Amsden <zach@...are.com>
To:	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: [PATCH] Add I/O hypercalls for i386 paravirt

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.

This patch is against 2.6.23-rc2-mm2, and should be targeted for 2.6.24.

Zach

View attachment "i386-mm-paravirt-io-ops.patch" of type "text/x-patch" (9004 bytes)