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SubjectRe: [PATCH 1/3] block: add queue flag for paravirt frontend drivers
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On Wed, 2008-11-05 at 10:20 +0100, Jens Axboe wrote: 
> On Tue, Nov 04 2008, Jeremy Fitzhardinge wrote:
> > Jens Axboe wrote:
> > >On Mon, Oct 27 2008, Fernando Luis Vázquez Cao wrote:
> > >
> > >>As is the case with SSD devices, we do not want to idle in AS/CFQ when
> > >>the block device is a paravirt front-end driver. This patch adds a flag
> > >>(QUEUE_FLAG_VIRT) which should be used by front-end drivers such as
> > >>virtio_blk and xen-blkfront to indicate a paravirtualized device.
> > >>
> > >
> > >All three patches look fine, although we could just reuse
> > >QUEUE_FLAG_NONROT directly. But I agree it makes sense to make the
> > >distinction, so I've just applied 1-3.
> > >
> >
> > I guess in theory you could imagine that the virtual device is mapped
> > directly onto a physical device, and the host OS does no scheduling, in
> > which case it would be appropriate for the guest do the work. But I
> > think otherwise this makes sense.
>
> For that specific case, it should just not set the flag.

When the virtual device is mapped directly onto a physical device the
host OS or the hypervisor could notify this fact to the guest using the
PCI configuration space (the bytes reserved for vendor-defined purposes
look like a good candidate to me). In such a case, on the guest side the
driver should just check the configuration space and set/unset the flag
accordingly.

The good thing about this approach is that it can be used with both
paravirt drivers and regular drivers (which is important for fully
virtualized guests).

I have just started to implement this idea but I would be great it you
let me know your take on this issue.

- Fernando

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