Re: commit "xen/blkfront: use tagged queuing for barriers"

From: Gerd Hoffmann
Date: Thu Aug 05 2010 - 13:46:52 EST

On 08/05/10 19:19, Christoph Hellwig wrote:
On Thu, Aug 05, 2010 at 10:08:44AM -0700, Jeremy Fitzhardinge wrote:
On 08/04/2010 09:44 AM, Christoph Hellwig wrote:
But either the blkfront patch is wrong and it needs to be fixed,
Actually both the old and the new one are wrong, but I'd say the new
one is even more wrong.

_TAG implies that the device can do ordering by tag. And at least the
qemu xen_disk backend doesn't when it advertizes this feature.

We don't use qemu at all for block storage; qemu (afaik) doesn't
have a blkback protocol implementation in it.

Upstream qemu has.

I'm guessing xen_disk
is to allow kvm to be compatible with Xen disk images?

No, is actually is a blkback implementation.

It certainly
isn't a reference implementation.

Indeed. I also havn't tested it for ages, not sure whenever it still works.

Disk images formats have nothing to do with the I/O interface. I
believe Gerd added it for running unmodified Xen guests in qemu,
but he can explain more of it.

Well, you can boot pv kernels with upstream qemu. qemu must be compiled with xen support enabled, you need xen underneath and xenstored must run, but nothing else (xend, tapdisk, ...) is required. qemu will call xen libraries to build the domain and run the pv kernel. qemu provides backends for console, framebuffer, network and disk.

There was also the plan to allow xen being emulated, so you could run pv kernels in qemu without xen (using tcg or kvm). Basically xenner merged into qemu. That project was never finished though and I didn't spend any time on it for at least one year ...

Hope this clarifies,

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