Re: [xen-devel][PATCH] PV driver compatibility

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2010-01-21 at 21:46 +0000, Keir Fraser wrote: 
> On 21/01/2010 20:38, "Ian Campbell" <Ian.Campbell@xxxxxxxxxxxxx> wrote:
> 
> >> You introduced the magic port value check, in xen-unstable:19964.
> > 
> > I'm guilty of pretty poor changelogging there, aren't I, I've no idea
> > how the unmodified drivers part of the change relates to the comment :-(
> 
> Yes, I wonder if it was even meant to be checked in. Or if it's an
> accidental merge of two patches both of which you intended to go in.

Yes, that's possible. I notice that I defined XEN_IOPORT_LINUX_PRODNUM
and XEN_IOPORT_LINUX_DRVVER but then hardcode 0xdead and 0xbeef where
they should be used, which suggests I wasn't quite ready to send the
patch...

> 
> I'm not sure unconditional unplug, as implemented, is a good idea. I can
> imagine people with setups in which emulated devices coexist with pv
> devices. Such a setup breaks if emulated devices all get unplugged when pv
> drivers load. Presumably in the environment this patch came from (Citrix
> XenServer) such a situation is disallowed, but I'm not so sure about
> proscribing it more generally.

The protocol allows for coarse grained selection of which devices to
unplug:
6) The drivers write a two-byte bitmask of devices to unplug to IO
   port 0x10.  The defined fields are:

   1 -- All IDE disks (not including CD drives)
   2 -- All emulated NICs
   4 -- All IDE disks except for the primary master (not including CD
        drives)

There is scope for extending this to a more explicit bitmask allowing
individual devices to be selected, if people are interested in that.

> Some people do want unplug though, so I think making it a non-default module
> option is a good idea.
> 
>  -- Keir
> 
> 



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