Re: [Xen-devel] [PATCH v3 6/9] xen/vpci: trap access to the list of PCI capabilities

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

On Tue, May 23, 2017 at 06:49:50AM -0600, Jan Beulich wrote:
> >>> On 27.04.17 at 16:35, <roger.pau@xxxxxxxxxx> wrote:
> > Add traps to each capability PCI_CAP_LIST_NEXT field in order to mask them 
> > on
> > request.
> > 
> > All capabilities from the device are fetched and stored in an internal list,
> > that's later used in order to return the next capability to the guest. Note
> > that this only removes the capability from the linked list as seen by the
> > guest, but the actual capability structure could still be accessed by the
> > guest, provided that it's position can be found using another mechanism.
> 
> Which is a problem. Drivers tied to a single device or a narrow set
> aren't unknown to do such. In fact in the past Intel has given us
> workaround outlines for some of their chipset issues which directed
> us to fixed offsets instead of using the capability chains.
> 
> > Finally the MSI and MSI-X capabilities are masked until Xen knows how to
> > properly handle accesses to them.
> > 
> > This should allow a PVH Dom0 to boot on some hardware, provided that the
> > hardware doesn't require MSI/MSI-X and that there are no SR-IOV devices in 
> > the
> > system, so the panic at the end of the PVH Dom0 build is replaced by a
> > warning.
> 
> While this is certainly nice for development / debugging purposes,
> what's the longer term intention with the functionality being added
> here? We had no need to mask capabilities for PV Dom0, so I would
> have hoped to get away without for PVH too.

Yes, this patch is mostly for development / debugging purposes, at
least in it's current state.

I though that maybe if users find issues with the MSI/MSI-X
implementations it would be easier to diagnose if there's an option to
disable those emulations.

Regarding what we would like to mask/hide from Dom0 I think the only
capability Xen must hide from Dom0 is ACS, because it's used by Xen
and Dom0 shouldn't poke at it at all (but that's an extended
capability anyway, which is not handled by this patch).

Of course for DomU Xen certainly wants to hide more capabilities, but
that's out of the picture ATM.

Let me know whether do you consider having this patch to mask
MSI/MSI-X capabilities on user request for Dom0 is helpful or not.

Thanks, Roger.

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