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

[Ryan Harper <ryanh@us.ibm.com>]

* Marc Haber <mh+qemu-devel@zugschlus.de> [2010-06-29 13:16]:
> 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?

We've got a sysfs 'serial' attribute for virtio-blk devices upstream[1].
I've got udev support for using this attribute to create disk/by-id (and
a fix for by-path) symlinks[2].  All that remains is to
re-spin/post the qemu virtio-blk serial patches[3] and get that in and
we'll have the full stack working as I've already tested libvirt (which
has disk serial support).


1. https://lists.linux-foundation.org/pipermail/virtualization/2010-June/015324.html
2. http://www.spinics.net/lists/hotplug/msg03931.html
3. http://lists.gnu.org/archive/html/qemu-devel/2010-03/msg01870.html



> 
> Greetings
> Marc
> 
> -- 
> -----------------------------------------------------------------------------
> Marc Haber         | "I don't trust Computers. They | Mailadresse im Header
> Mannheim, Germany  |  lose things."    Winona Ryder | Fon: *49 621 72739834
> Nordisch by Nature |  How to make an American Quilt | Fax: *49 3221 2323190

-- 
Ryan Harper
Software Engineer; Linux Technology Center
IBM Corp., Austin, Tx
ryanh@us.ibm.com