Hi Volodymyr,
On 16/11/2023 16:53, Volodymyr Babchuk wrote:
Julien Grall <jul...@xen.org> 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:
Are you talking about HVM or PV? I am not very familiar for the latter
but for the former, QEMU is today responsible to find a free slot.
You can also specify which virtual slot you want to use in XL:
=item B<vslot>=I<NUMBER>
=over 4
=item Description
Specifies the virtual slot (device) number where the guest will see this
device. For example, running L<lspci(1)> in a Linux guest where B<vslot>
was specified as C<8> would identify the device as C<00:08.0>. Virtual
domain
and bus numbers are always 0.
"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 am not well-versed with the PCI specification. However, what the specs
says and what users do tend to be different :).
I know a few cases where you need to specify the slot. I have mentioned
one earlier with the Intel Graphic cards.
I mean, we can make toolstack responsible of assigning BDFs, but this
might break existing x86 setups...
It is not clear to me how letting the toolstack selecting the vslot
would be a problem for HVM (see above). However, it is clear that you
may break some x86 setup if always allocate a "random" slot.
Cheers,
--
Julien Grall
