Re: [Xen-devel] Re: IOMMU faults

[Jean Guyader <jean.guyader@xxxxxxxxxxxxx>]

On 16/06 11:07, Tim Deegan wrote:
> Hi Jean, 
> 

Reply below,

> At 10:47 +0100 on 16 Jun (1308221277), Jean Guyader wrote:
> > We have seed such behavior when we were testing GPU assignement especially
> > the Intel GPU. The problem is that domain destruction in Xen is 
> > assynchronous
> > and right now the pci device reset is done in dom0 with some help of the 
> > toolstack.
> > 
> > In the Intel GPU case we need to make sure that the guest memory and the 
> > IOMMU
> > are still in place while we perform to reset otherwise the device drift into
> > an unstable state.
> > 
> > There is probably other ways to do that in a cleaner way but we decided to 
> > move
> > the pci reset code into Xen, so we are sure we perform the reset while the 
> > device
> > is in a known state (functionning state).
> > 
> > Attached is the patch we have in XenClient that move the pci reset into Xen.
> 
> Thanks, Jean.  This sounds like a good idea to me, though I'd like to
> hear Wei and Allen's opinions.
> 
> The patch is incomplete (missing the new pci_reset.[ch] files) but I get
> the general idea.  A few questions:

Reattach the full patch.

> 
>  - Why the special handling for one graphics device on each domain? 
>    (And if one, why not all?)

No good reason really just a limitation of the patch, we can trivially
get ride of the limitation.

>  - Why not reset when the target is dom0?  It seems like it can do no
>    harm and should protect dom0 from assigning itself an active PCI
>    card. 

Reset could be quiet expensive (couple of seconds sometimes).
We did that to avoid a double reset on domain reboot.
I agree that we should remove that, or extend the IOMMU API
so we can reassign from domU to domU without going through
dom0.

> 
> Of course, even with this patch, my original question still stands:
> should Xen do something more assertive in the IOMMU fault handler?
> 

What we really want to achive here is to stop DMA on this device.
One way of doing it is to perform a proper PCI reset (FLR, secondary
bus reset, ...) when that happens.

Jean

Attachment: iommu-vtd-pci-reset-on-reassign
Description: Text document

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