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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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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