linux-kernel - Re: [KVM PATCH v4 3/3] kvm: add iosignalfd support

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Message-ID: <4A1DA653.4050109@gmail.com>
Date:	Wed, 27 May 2009 16:45:07 -0400
From:	Gregory Haskins <gregory.haskins@...il.com>
To:	Mark McLoughlin <markmc@...hat.com>
CC:	Gregory Haskins <ghaskins@...ell.com>, Avi Kivity <avi@...hat.com>,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Davide Libenzi <davidel@...ilserver.org>, mtosatti@...hat.com
Subject: Re: [KVM PATCH v4 3/3] kvm: add iosignalfd support

Mark McLoughlin wrote:
> On Wed, 2009-05-27 at 13:40 -0400, Gregory Haskins wrote:
>   
>> Mark McLoughlin wrote:
>>     
>>> On Wed, 2009-05-27 at 15:11 +0300, Avi Kivity wrote:
>>>
>>>   
>>>       
>>>> Multiple cookies on the same address are required by virtio.  You can't 
>>>> mux since the data doesn't go anywhere.
>>>>
>>>> Virtio can survive by checking all rings on a notify, and we can later 
>>>> add a mechanism that has a distinct address for each ring, but let's see 
>>>> if we can cope with multiple cookies.  Mark?
>>>>     
>>>>         
>>> Trying to catch up, but you're talking about replacing virtio-pci
>>> QUEUE_NOTIFY handling with iosignalfd ?
>>>
>>> For a perfect replacement, what you really need is to be able to
>>> register multiple cookies per address range, but only have them trigger
>>> if the written data matches a provided value.
>>>   
>>>       
>> Hmm..thats an interesting idea.  To date, the "cookie" has really been
>> for identifying the proper range selected for deassignment.  I never
>> thought of using it as an actual trigger value at run-time.
>>
>>     
>>> If the data is lost, virtio has no way of knowing which queue is being
>>> notified, so we either end up with per-device, rather than per-queue,
>>> notifications (probably not too bad for net, at least) or a different
>>> notify address per queue (limiting the number of queues per device).
>>>   
>>>       
>> The addr-per-queue is how I was envisioning it, but the trigger value
>> concept hadn't occurred to me.  I could make this an option during
>> assignment (e.g. "COOKIE" flag means only trigger on writes of the
>> provided cookie, otherwise trigger on any write).  Sound good?
>>     
>
> Ah, I'd been thinking of the trigger data being provided separately to
> the cookie.
>   

Yeah, that shouldn't be a problem.  Its probably cleaner, too.  Perhaps
I can simplify things and make the trigger width fixed (say, always
32bit) just so I dont have to support 8,16,32 and 64 variants.

> The virtio ABI is fixed, so we couldn't e.g. have the guest use a cookie
> to identify a queue - it's just going to continue using a per-device
> queue number. 

Actually, I was originally thinking this would be exposed as a virtio
FEATURE bit anyway, so there were no backwards-compat constraints.  That
said, we can possibly make it work in a backwards compat way, too. 
IIRC, today virtio does a PIO cycle to a specific register with the
queue-id when it wants to signal guest->host, right?  What is the width
of the write?


> So, if the cookie was also the trigger, we'd need an
> eventfd per device.
>   

I'm having trouble parsing this one.  The cookie namespace is controlled
by the userspace component that owns the corresponding IO address, so
there's no reason you can't make "queue-id = 0" use cookie = 0, or
whatever.  That said, I still think a separation of the cookie and
trigger as suggested above is a good idea, so its probably moot to
discuss this point further.

-Greg



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