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

Gregory Haskins
2009-05-27T20:45:09Z

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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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