Re: [Xen-devel] RFC: Automatically making a PCI device assignable in the config file

[Gordan Bobic <gordan@xxxxxxxxxx>]

On Wed, 10 Jul 2013 16:29:33 +0100, George Dunlap 
<george.dunlap@xxxxxxxxxxxxx> wrote:
> On 10/07/13 16:12, Gordan Bobic wrote:
> > On Wed, 10 Jul 2013 15:45:49 +0100, George Dunlap 
> > <george.dunlap@xxxxxxxxxxxxx> wrote:
> > > On 10/07/13 14:55, Ian Jackson wrote:
> > > > George Dunlap writes ("Re: [Xen-devel] RFC: Automatically making a 
> > > > PCI device assignable in the config file"):
> > > > > Auto-seizing is fairly dangerous; you could easily accidentally 
> > > > > yank out
> > > > > the ethernet card, or even the disk that dom0 is using.  I really 
> > > > > think
> > > > > it should have to be enabled on a device-by-device basis.
> > > > I don't think this makes any sense.
> > > >
> > > > In practice you are saying that in order to avoid mistakes, the 
> > > > local
> > > > admin must provide the BDF of the device to be passed through in 
> > > > two
> > > > separate places.
> > > That's not what I had in mind; what I had in mind was something 
> > > like this:
> > > pci = [ '08:04.1,seize=1' ]
> > >
> > > Or alternately:
> > >
> > > xl pci-attach h0 08:04.1,seize=1
> > >
> > > One could also imagine having something in xl.conf like the 
> > > following:
> > > pci.autoseize = [ '08:04.1','01:00.0' ]
> > >
> > > In this case you wouldn't be simply copy and pasting, because you'd
> > > probably have different domains handling each device.  But in any
> > > case, this was just exploring the alternatives -- I don't think 
> > > that's
> > > the best thing to do.
> > >
> > > > But that doesn't actually help.  If this is all properly 
> > > > documented
> > > > and so forth (ie, if the admin has a smooth experience and doesn't
> > > > trip over having dailed to "seize" the device), they will just
> > > > cut-and-paste the same value into both places in the config.
> > > >
> > > > They will also mutter under their breath to ask why this is
> > > > necessary...
> > > If someone can accidentally type "xl pci-attach 08:04.0" instead of
> > > "xl pci-attach 08.04.1" and suddenly completely lose their network
> > > connectivity (or yank their hard drive out), then I think they'll
> > > appreciate it.
> > >
> > > In any case, at the moment it's much worse: you have to either
> > > scriptwise run "xl pci-assignable-add" a device, or add an 
> > > exclusion
> > > on the Linux commandline in grub.  So far I'm the only person I 
> > > know
> > > who has complained about it. :-)
> > This would be an awesome feature, but success can be mixed. My
> > experience is that Nvidia drivers (as one example, I'm sure there
> > are others) don't handle device detaching very gracefully. ACS
> > might help but I have no means of testing
> > that at the moment. So ultimately, you still have to either
> > exclude at least some devices from grub (if
> > xen-pciback is built in) or in modprobe.d (attach device to
> > pciback in driver pre-install). I'm not sure what can be
> > done about this considering we are talking about 3rd
> > party binary drivers. :(
> Yes, unfortunately we'll always have to have the fallback for devices
> that don't play well with being re-assigned.  But I my very limited
> experience with network cards gives me some hope that at least for
> many devices for which driver domains might be in use, the
> auto-reassign thing will be useful in a number of cases.
What about having a feature that upon parsing the config file modifies
the grub.conf or modprobe.d/xen-pciback.conf (depending on whether
xen-pciback is module or built in) and adds the required entries
for the device(s)?

Gordan

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