Re: [Xen-devel] [Qemu-devel] [PATCH] Add Xen platform PCI device version 2.

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Tim Deegan [mailto:tim@xxxxxxx]
> Sent: 26 June 2013 12:58
> To: Paul Durrant
> Cc: Ian Campbell; Matt Wilson; Alex Bligh; xen-devel@xxxxxxxxxxxxx; qemu-
> devel@xxxxxxxxxx
> Subject: Re: [Xen-devel] [Qemu-devel] [PATCH] Add Xen platform PCI device
> version 2.
> 
> At 11:23 +0000 on 26 Jun (1372245783), Paul Durrant wrote:
> >  We could blacklist all existing Citrix PV drivers in upstream QEMU,
> > to avoid the clash, but that seems like a very unfriendly
> > approach. Also, it's not going to stop someone with an existing VM,
> > who happens to be using legacy Citrix PV drivers (an AWS VM for
> > instance) receiving a driver from Windows Update that will blue-screen
> > their VM on next reboot. Hence the only way forward is to bind the new
> > drivers to something new, that we can control, so we know what driver
> > a VM is going to get from Windows Update.
> 
> I don't think you ever got an answer about whether this could be
> finagled using version numbers in the drivers.

No, we thought about that and I don't believe it would be possible.
 
> Also: would it not be possible to have a blkfront driver (even a trivial
> one) in your new bus driver so it can detect and avoid this problem?
> 
> Or: have your bus driver detect when the blkfront driver is missing and
> not unplug the emulated disk?  In fact wouldn't having the emulated disk
> driver do the unplug solve it for free?
>

The issue is the old s/w not the new s/w. The old drivers make assumptions 
about each other's presence as we can't change that because they are already 
out there.
 
> > And we may indeed need to
> > modify its revision in future so that we can retire old sets of PV
> > drivers and replace them with new ones, but only for newer XenServer
> > releases. Thus, I also propose to make the PCI revision of the new
> > device a command line parameter.
> 
> I'd rather not.  This gets straight back to having host-admin controls
> that have to manually be matched to in-guest software.
> 

Well not really. This is just the same as a h/w vendor shipping a new device. 
The drivers for the old device are still there on Windows Update; so no change. 
The new drivers are for the new device so only download when it is present. 
Setting the revision number for the 'citrix pv bus' device would only be like 
choosing which emulated NIC you want.

  Paul

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