linux-kernel - Re: [PATCH 0/16 v6] PCI: Linux kernel SR-IOV support

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Message-ID: <492044A7.3080107@redhat.com>
Date:	Sun, 16 Nov 2008 18:04:55 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Anthony Liguori <anthony@...emonkey.ws>
CC:	Andi Kleen <andi@...stfloor.org>,
	"randy.dunlap@...cle.com" <randy.dunlap@...cle.com>,
	"grundler@...isc-linux.org" <grundler@...isc-linux.org>,
	"Chiang, Alexander" <achiang@...com>,
	Matthew Wilcox <matthew@....cx>, Greg KH <greg@...ah.com>,
	"rdreier@...co.com" <rdreier@...co.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"jbarnes@...tuousgeek.org" <jbarnes@...tuousgeek.org>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>
Subject: Re: [PATCH 0/16 v6] PCI: Linux kernel SR-IOV support

Anthony Liguori wrote:
> I don't think it's established that PV/VF will have less latency than 
> using virtio-net.  virtio-net requires a world switch to send a group 
> of packets.  The cost of this (if it stays in kernel) is only a few 
> thousand cycles on the most modern processors.
>
> Using VT-d means that for every DMA fetch that misses in the IOTLB, 
> you potentially have to do four memory fetches to main memory.  There 
> will be additional packet latency using VT-d compared to native, it's 
> just not known how much at this time.

If the IOTLB has intermediate TLB entries like the processor, we're 
talking just one or two fetches.  That's a lot less than the cacheline 
bouncing that virtio and kvm interrupt injection incur right now.

-- 
error compiling committee.c: too many arguments to function

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