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

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Message-ID: <4916DD31.4000503@redhat.com>
Date:	Sun, 09 Nov 2008 14:53:05 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Anthony Liguori <anthony@...emonkey.ws>,
	Matthew Wilcox <matthew@....cx>,
	"Fischer, Anna" <anna.fischer@...com>, Greg KH <greg@...ah.com>,
	H L <swdevyid@...oo.com>,
	"randy.dunlap@...cle.com" <randy.dunlap@...cle.com>,
	"grundler@...isc-linux.org" <grundler@...isc-linux.org>,
	"Chiang, Alexander" <achiang@...com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"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>,
	"mingo@...e.hu" <mingo@...e.hu>
Subject: Re: [PATCH 0/16 v6] PCI: Linux kernel SR-IOV support

Andi Kleen wrote:
> Anthony Liguori <anthony@...emonkey.ws> writes:
>   
>> What we would rather do in KVM, is have the VFs appear in the host as
>> standard network devices.  We would then like to back our existing PV
>> driver to this VF directly bypassing the host networking stack.  A key
>> feature here is being able to fill the VF's receive queue with guest
>> memory instead of host kernel memory so that you can get zero-copy
>> receive traffic.  This will perform just as well as doing passthrough
>> (at least) and avoid all that ugliness of dealing with SR-IOV in the
>> guest.
>>     
>
> But you shift a lot of ugliness into the host network stack again.
> Not sure that is a good trade off.
>   

The net effect will be positive.  We will finally have aio networking 
from userspace (can send process memory without resorting to 
sendfile()), and we'll be able to assign a queue to a process (which 
will enable all sorts of interesting high performance things; basically 
VJ channels without kernel involvement).

> Also it would always require context switches and I believe one
> of the reasons for the PV/VF model is very low latency IO and having
> heavyweight switches to the host and back would be against that.
>   

It's true that latency would suffer (or alternatively cpu consumption 
would increase).

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

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