Re: [Xen-users] Simple Query on PCI passthrough I/O

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> > > > * You need to be running a dom0 kernel in the guest with the PCI
>
> card.
>
> > > Whoa! I did not know this either. Again what are the reasons? Kindly
>
> let > me
>
> > > know.
> >
> > Regarding the kernel version.  You don't necessarily need to be
> > running the same kernel as dom0.  But you do need to be running
> > a dom0-capable kernel because domU-only kernels lack hardware support.
>
> Hmm, I thought that meant the domU kernel had to run the pcifront driver
> plus the native driver for the PCI device.  Is that right or have I
> missed something?

I had in mind that some of the privileged operations (i.e. included in dom0 
kernels not in purely unprivileged domU kernels) were required for a domain 
to support a native device driver correctly and so you'd need a dom0-enabled 
kernel.  I *might* be wrong nowadays but I wouldn't be surprised if it were 
still the case.  Real device drivers use operations that aren't necessary for 
a purely unprivileged domU kernel to support, after all.

So in conclusion, it *might* be possible to add pcifront and a native device 
driver to an otherwise pure domU kernel but I wouldn't necessarily expect it 
to work.  Hence I recommend using a dom0 /capable/ kernel in a domU that owns 
a PCI device.  Not necessarily the same kernel as dom0, although may be 
easiest to set up on some installs.

This distinction is less relevant these days since many distributions ship a 
single combined dom0 and domU kernel for all purposes.

> Is there a way to tell if the domU is actually running pcifront?  My
> /boot/config-2.6.18-53.el5xen shows CONFIG_XEN_PCIDEV_FRONTEND=y, but is
> there a way to verify that it is actually in the running kernel?

Well, if it's =y then I expect it should be in there.  You can probably check 
this by looking for output in dmesg and by looking for an entry in /sysfs.  I 
don't have a Xen system handy at this moment to check the exact details for 
you, sorry.

Hope that helps,

Cheers,
Mark

-- 
Push Me Pull You - Distributed SCM tool (http://www.cl.cam.ac.uk/~maw48/pmpu/)

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