Re: [RFC PATCH 2/6] xen/public: arch-arm: reserve resources for virtio-pci

[Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>]

Hi Julien,

Julien Grall <julien@xxxxxxx> writes:

> On 16/11/2023 15:26, Stewart Hildebrand wrote:
>> On 11/16/23 10:12, Julien Grall wrote:
>>> Hi Volodymyr,
>>>
>>> On 16/11/2023 15:07, Volodymyr Babchuk wrote:
>>>> With my vPCI patch series in place, hypervisor itself assigns BDFs for
>>>> passed-through devices. Toolstack needs to get this information to know
>>>> which BDFs are free and can be used by virtio-pci.
>>>
>>> It sounds a bit odd to let the hypervisor to assign the BDFs. At
>>> least because there might be case where you want to specific vBDF
>>> (for instance this is the case with some intel graphic cards). This
>>> should be the toolstack job to say "I want to assign the pBDF to
>>> this vBDF".
>> Keep in mind we are also supporting dom0less PCI passthrough.
> Right, but even with that in mind, I expect the Device-Tree to provide
> the details where a given PCI is assigned.
>
> You could have logic for assigning the BDF automagically. But that
> should be part of dom0less, not deep into the vPCI code.

As far as I know, right now toolstack does not allow you to assign BDF
in any form. For x86 there are two options, and they are controlled by
"passthrough" option of xen-pciback driver in Linux:

  "Option to specify how to export PCI topology to guest:"
  " 0 - (default) Hide the true PCI topology and makes the frontend"
  "   there is a single PCI bus with only the exported devices on it."
  "   For example, a device at 03:05.0 will be re-assigned to 00:00.0"
  "   while second device at 02:1a.1 will be re-assigned to 00:01.1."
  " 1 - Passthrough provides a real view of the PCI topology to the"
  "   frontend (for example, a device at 06:01.b will still appear at"
  "   06:01.b to the frontend). This is similar to how Xen 2.0.x"
  "   exposed PCI devices to its driver domains. This may be required"
  "   for drivers which depend on finding their hardward in certain"
  "   bus/slot locations.");

Also, isn't strict dependency on BDF breaks the PCI specification? I
mean, of course, you can assign Function on random, but what about Bus
and Device parts?


I mean, we can make toolstack responsible of assigning BDFs, but this
might break existing x86 setups... 

-- 
WBR, Volodymyr