Re: [Qemu-devel] virtio block device and sysfs

[Marc Haber]

Hi,

I had lost this topic for more than a few weeks...

On Mon, Mar 22, 2010 at 10:59:20AM -0400, john cooper wrote:
> Marc Haber wrote:
>> On Mon, Mar 22, 2010 at 02:26:11AM -0400, john cooper wrote:
>>> This is a tad ironic as that is how this saga begun.  Namely stuffing
>>> 20 bytes of serial number string into the virtio-blk PCI config space
>>> on qemu's side and pushing it over to the guest driver.  I exposed this
>>> to the guest app via a new ioctl cmd which itself was the original
>>> point of contention.  Someone took issue with introducing a new
>>> interface citing the existence of ATA and SCSI counterparts.  However
>>> dragging in the associated baggage in order to emulate those interfaces
>>> unintentionally bloated usage of the config space to the point of breakage.
>>> To address this I'd moved from using config space to an unused BAR which
>>> (understandably) didn't go over too well.  Somewhere along the line Rusty
>>> posted a minimal alternative version which directly used a virtio request
>>> to retrieve the data from qemu which is arguably the right way to do the
>>> job.
>>
>> *argh* That sounds like politics.
>
> Well, maybe.  But it is hard to argue with anyone calling the
> ATA_IDENTIFY interface ugly -- it is.

Indeed. It was designed to interact with Hardware, not with software
trying to look like hardware.

> Concerning qemu->virtio_blk communication, I don't think an
> argument to use PCI config space exists after one looks at
> the implementation of adding a new virtio request.

Indeed. Frankly, I don't care too much about how the information is
passed into the guest as long as the guest can read what the host
said. (mis)using ATA data was only a naive approach since I know that
there is an existing interface. If there is a better one, even better.

>>> That said we still had a dispute over what interface would be used to
>>> pass the S/N back to the guest: a new interface or reuse of an existing
>>> interface (eg: ATA IDENTIFY).  That's where things fizzled when we
>>> couldn't immediately resolve the issue.  So publishing the S/N in
>>> /sys would seem to side step this snag.
>>
>> Re-using an existing interface would probable make it easier for
>> non-Linux OSses to also take advantage of this, since their ATA driver
>> is already there.
>
> Exactly my singular motivation and honestly the only redeeming
> quality of propagating that crusty interface.

But otoh, I will only use Linux on the guest side, and it is
motivation for other developers to build a virtio driver for their
favorite OS.

>>> I could have swore I sent out a guest-driver-app-interface-less
>>> version of the patch using virtio to pass the S/N but didn't find it in
>>> the archives.  I did however locate it and can bring it forward as a
>>> reference for the above if interest exists.
>>
>> If it brings the issue forward and gives me hope to be able to do what
>> I want to do in a reasonable time frame, why not.
>
> Just time.  But I'll resurrect the patch so we don't have to go through
> all of this again.

Did that work?

Greetings
Marc

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