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

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Message-ID: <49D5ED11.2000800@redhat.com>
Date:	Fri, 03 Apr 2009 14:03:45 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Gerd Hoffmann <kraxel@...hat.com>
CC:	Herbert Xu <herbert@...dor.apana.org.au>, 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

Gerd Hoffmann wrote:
> Avi Kivity wrote:
>   
>> There is no choice.  Exiting from the guest to the kernel to userspace
>> is prohibitively expensive, you can't do that on every packet.
>>     
>
> I didn't look at virtio-net very closely yet.  I wonder why the
> notification is that a big issue though.  It is easy to keep the number
> of notifications low without increasing latency:
>
> Check shared ring status when stuffing a request.  If there are requests
> not (yet) consumed by the other end there is no need to send a
> notification.  That scheme can even span multiple rings (nics with rx
> and tx for example).
>   

If the host is able to consume a request immediately, and the guest is 
not able to batch requests, this breaks down.  And that is the current 
situation.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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