Re: [Qemu-devel] VFIO PCIe Extended Capabilities

[Alex Williamson]

On Tue, 19 Jul 2016 21:16:40 +0300
Marcel Apfelbaum <address@hidden> wrote:

> On 07/19/2016 08:55 PM, Alex Williamson wrote:
> > On Tue, 19 Jul 2016 11:22:29 -0600
> > Alex Williamson <address@hidden> wrote:
> >  
> >> On Tue, 19 Jul 2016 17:12:45 +0000
> >> Spenser Gilliland <address@hidden> wrote:
> >>  
> >>> Hi,
> >>>
> >>> I noticed your patches "vfio: add pcie extended capability support" and 
> >>> "vfio/pci: Hide SR-IOV capability" have gone into qemu mainline.  These 
> >>> look really good, and thanks so much for doing these.
> >>>
> >>> I was wondering if there were any side effects to removing the 
> >>> pci_bus_is_express check on line 1776 of hw/vfio/pci.c .
> >>>
> >>>      /* Only add extended caps if we have them and the guest can see them 
> >>> */
> >>> -   if (!pci_is_express(pdev) || !pci_bus_is_express(pdev->bus) ||
> >>> +  if (!pci_is_express(pdev) ||
> >>>          !pci_get_long(pdev->config + PCI_CONFIG_SPACE_SIZE)) {
> >>>          return 0;
> >>>      }
> >>>
> >>> I'm asking because it looks like the defaults for libvirt/OpenStack are 
> >>> to create a "hostdev" stanza in the libvirt xml to define this pass 
> >>> through condition.  However, it also appears that the "hostdev" stanza 
> >>> only supports pci-bridge bus connections.  Thus, it's not easily possible 
> >>> to use this patch in a libvirt/OpenStack environment as the bus is 
> >>> technically a non-express bus.  It looks like adding PCIe bus support to 
> >>> libvirt/OpenStack may be a lot more effort than a simple workaround here.
> >>>
> >>> I have tested this on my local system and it does work as intended for my 
> >>> use case.  The following is from an OpenStack VM and shows that the 0x340 
> >>> extended configuration space is passed through correctly.  I've also done 
> >>> testing which uses this space and the results are positive.  
> >>
> >> If the bus is not express then extended capabilities on the device
> >> should not be accessible, this would be a QEMU bug for allowing it.
> >> Cc'ing Marcel for that.  Thanks,  
> 
> Hi Spencer,
> 
> Indeed, if a device is attached to a PCI bus it makes no sense to advertise 
> the extended configuration space.
> Can you please share the QEMU command line? Maybe is possible to make the 
> device's bus PCIe in QEMU?

I think that any instance of a q35 machine where the assigned device is
placed on the conventional PCI bridge will create this scenario.  It's
the default for attaching devices to a libvirt managed q35 VM AFAIK.

> >
> > In fact, I've tried to fix this multiple times:
> >
> > https://lists.nongnu.org/archive/html/qemu-devel/2015-10/msg05384.html
> > https://lists.nongnu.org/archive/html/qemu-devel/2015-11/msg02422.html
> > https://lists.nongnu.org/archive/html/qemu-devel/2016-01/msg03259.html
> >
> > Yet the patch remains unapplied :(  
> 
> I thought is it in already. Maybe Michael can add it as part of the hard 
> freeze.
> And if the patch will be applied, the tweak above wouldn't help, right Alex?

The tweak Spenser suggested above should be unnecessary with my proposed
patch applied.  Only now searching for that patch did I notice Michael's
comment hidden at the bottom of his reply, which I assume is why it
never got applied:

https://patchwork.kernel.org/patch/8057411/

Anyway, the current behavior is clearly a bug, so QEMU hard freeze
should be irrelevant.  If anyone wants to take over the patch, feel
free.  Thanks,

Alex