Re: RFC - PV blk driver for hvm guest

[Steven Smith <sos22-xen@srcf.ucam.org>]

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> As I look further into this, an over-arching question begins to
> form.  Originally, I had suggested that a 'type=' be added to the
> 'disk' line, like the net, to indicate the use of FV or PV drivers.
> But now I'm wondering if a more global tag should be used to
> indicate that all drivers are either FV or PV.  Implementing the
> original idea would be a more involved but create the opportunity
> for one disk device to be supported by a FV driver and another by a
> PV driver.  The question is, why would anyone ever wish to have
> mixed driver technology?  With the PV drivers having such a profound
> increase in performance and given that PV drivers are available for
> the quest (and assuming if a a PV driver is available for one
> technology, i.e. LAN or BLK, then it would be available for the
> other) then why would someone use the FV driver? Thus the more
> simple solution to have a single switch in the guest's config file
> indicating the mode for all devices, LAN or BLK.
Well, it's rather handy when you're actually working on the PV
drivers, since you can e.g. have an ioemu root device and test the
block driver on some other filesystem.  Plus, if at some point in the
future we implement a PV framebuffer you're likely to have some guest
installs with BLK+LAN but not framebuffer, and so mixed mode would be
useful there.

Steven.

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