netdev - Re: [RFC PATCH 00/17] virtual-bus

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Message-ID: <49D47F11.6070400@redhat.com>
Date:	Thu, 02 Apr 2009 12:02:09 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
CC:	ghaskins@...ell.com, anthony@...emonkey.ws, andi@...stfloor.org,
	linux-kernel@...r.kernel.org, agraf@...e.de, pmullaney@...ell.com,
	pmorreale@...ell.com, rusty@...tcorp.com.au,
	netdev@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [RFC PATCH 00/17] virtual-bus

Herbert Xu wrote:
> Avi Kivity <avi@...hat.com> wrote:
>   
>> virtio is already non-kvm-specific (lguest uses it) and non-pci-specific 
>> (s390 uses it).
>>     
>
> I think Greg's work shows that putting the backend in the kernel
> can dramatically reduce the cost of a single guest->host transaction.
> I'm sure the same thing would work for virtio too.
>   

Virtio suffers because we've had no notification of when a packet is 
actually submitted.  With the notification, the only difference should 
be in the cost of a kernel->user switch, which is nowhere nearly as 
dramatic.

>> If you have a good exit mitigation scheme you can cut exits by a factor 
>> of 100; so the userspace exit costs are cut by the same factor.  If you 
>> have good copyless networking APIs you can cut the cost of copies to 
>> zero (well, to the cost of get_user_pages_fast(), but a kernel solution 
>> needs that too).
>>     
>
> Given the choice of having to mitigate or not having the problem
> in the first place, guess what I would prefer :)
>   

There is no choice.  Exiting from the guest to the kernel to userspace 
is prohibitively expensive, you can't do that on every packet.

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

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