Re: [Xen-devel] q35 in xen? vfio in xen?

["Zhang, Eniac" <eniac-xw.zhang@xxxxxx>]

Hi Konrad,

Here's what I see when start a VM under xen using pciback to pass a pci-e 
device into domU.  The device can be seen by guest, and also functioning fine.  
But it's not seen as a pci-e device, rather, it looks just like an ordinary pci 
device because only the first 0x100 bytes of its configuration space is 
accessible.  So if a driver needs to use data in the extended configuration 
space for certain features, it will fail.

When you say you "did PCIe pass through of an VF of an SR-IOV device".  Are you 
actually using it as a pci-e device or have throttled it back to pci mode 
without aware of the difference?  If you did see the pci-e device in guest, can 
you share your xl.cfg file as well as lspci/lspci -t/lspci -xxxx output from 
guest?

Also to echo your second comment:  I might still be a newbie in qemu field (I 
started working on this 4 months ago).  I thought the chipset limits what you 
can see/do in vm.  Ie.  If you have 440fx emulations then you can't have any 
pci-e devices (fake or passthru) in the same system.  Is that not true?

Regards/Eniac

-----Original Message-----
From: Konrad Rzeszutek Wilk [mailto:konrad.wilk@xxxxxxxxxx] 
Sent: Friday, February 21, 2014 5:32 PM
To: Zhang, Eniac
Cc: xen-devel@xxxxxxxxxxxxx
Subject: RE: [Xen-devel] q35 in xen? vfio in xen?


On Feb 21, 2014 4:58 PM, "Zhang, Eniac" <eniac-xw.zhang@xxxxxx> wrote:
>
> Hi Konrad, 
>
> Thanks for your reply. 
>
> Yes, I am aware of the pciback. Unfortunately it doesn't seem to support 
> pci-e passthrough. (I could be wrong here)

I just did PCIe pass through of an VF of an SR-IOV device. It certainly is PCIe.

>
> There are two reason that I am interested in this. For one, my project calls 
> for pci-e device passthrough, which can't be accomplished with 440fx chipset 
> emulation. Secondly, I feel we ought to move on with the technology. 440fx 
> is ancient in computer terms. Qemu is good and all that, but if it refuses 
> to support pci-e natively then it's just a matter of time that it will become 
> obsoleted. The trend is clear that pci-e is taking over the world. 
>

I am not sure what you are saying but it does not matter whether QEMU emulates 
440fx or q35 for PCI pass through .

> Regards/Eniac 
>
> -----Original Message----- 
> From: Konrad Rzeszutek Wilk [mailto:konrad.wilk@xxxxxxxxxx] 
> Sent: Friday, February 21, 2014 2:50 PM 
> To: Zhang, Eniac 
> Cc: xen-devel@xxxxxxxxxxxxx 
> Subject: Re: [Xen-devel] q35 in xen? vfio in xen? 
>
> On Fri, Feb 21, 2014 at 09:41:39PM +0000, Zhang, Eniac wrote: 
> > Hi all, 
> > 
> > I am playing with q35 chipset in qemu (1.6.1). It seems we can't enable 
> > q35 machine under xen yet. I made a few quick hacks which all fail 
> > miserably (linux kernel oops and window BSOD). I was wondering why this 
> > hasn't been done (q35 was introduced into qemu in 2009). 
> > 
> > Next question, vfio works very well for me in standalone qemu (with Linux 
> > host handling iommu), but is that supported under xen? I haven't tried 
> > anything there yet because my gut-feeling is that it won't work. Because 
> > passing vfio device to qemu can only be done on qemu commandline, and xen 
> > is not aware of this passing through device, thus not able to make iommu 
> > arrangement for this device. Am I on the right track here? 
>
> Yes and no. VFIO won't work - but QEMU does do PCI passthrough under Xen. It 
> uses a different mechanism (and you need to bind the device to pciback). 
>
> > 
> > I am interested in implementing both these two features. I'd like to 
> > connect with anyone who's already on this so we don't duplicate the 
> > efforts. 
>
> What do you need Q35 for? 
>
> > 
> > Regards/Eniac 
>
> > _______________________________________________ 
> > Xen-devel mailing list 
> > Xen-devel@xxxxxxxxxxxxx 
> > http://lists.xen.org/xen-devel 
>
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