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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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