Xen devel - Oct 2006 - Re: RFC - PV blk driver options for hvm guest

>>>> Steven Smith <sos22-xen@srcf.ucam.org> 10/01/06 11:42 AM
>>>
>> My testing of the the PV blk driver was first done by creating a
>> second blk device in the ''disk='''' line of
the config file.  I could
>> not use the PV driver for the first device because the guest''s
ide
>> driver lays claim on hda before the PV driver is loaded.  In an
>> attempt to get the guest to come up entirely on the PV drivers I
>> modified qemu''s ide.c to make the emulated device incompatible
with
>> the native IDE driver.  This allows the PV driver to create hda and
>> the OS came up just fine.  (I had also rebuilt the guest''s
initrd to
>> include and load the PV drivers, of course.)
> And the bootloader coped with this without any objections?  I suppose
> they''d mostly be using BIOS services, and I could well believe our
> BIOS doesn''t actually bother to check the PCI device ids before
going
> at the IDE controller.
There were no apparent objections to booting on the PV blk driver.
Needless to say, I was quite pleased, as well, at how much more rapidly the
the guest OS came up.


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.

Having said all this, it occurs to me that this idea may be a bit premature as
qemu''s emulation of the ide device has sufficient support for removable
media, whereas PV blk drivers have very little, therefore, being able to
indicate which blk devices use either PV of FV drivers is warranted, at least
for now.

-- Ross




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