Re: [Qemu-devel] [PATCH] virtio-pci: Don't force Subsystem Vendor ID = Vendor ID

[Vadim Rozenfeld <vrozenfe@redhat.com>]

On 03/11/17 19:23, Ladi Prosek wrote:
> On Fri, Nov 3, 2017 at 8:20 AM, Gerd Hoffmann <kraxel@redhat.com> wrote:
>>>>> Signed-off-by: Ladi Prosek <lprosek@redhat.com>
>>>> I wonder whether it's a problem that legacy devices ignore
>>>> the subsystem ID (that's part of spec).
>>> I don't understand this comment. I don't see anything in the spec
>>> related to ignoring the subsystem ID.
>> Well, the subsystem *device* id is defined to be the virtio device id,
>> so it is certainly not ignored.  The subsystem *vendor* id is not used
>> as far I know (or ignored in the sense that it doesn't change driver
>> behavior), allowing to set that makes sense to me.
> Yes, thanks, I'm assuming that Michael meant the subsystem device ID.
> The PCI spec seems to be using "subsystem ID" for the name of this
> field. I understand that this ID must not change in legacy devices.
>
> Interestingly, Windows appears to allow matching drivers only on the
> full subsystem device ID + subsystem vendor ID 32-bit value, not on
> only one of the two.
>
> PCI\VEN_v(4)&DEV_d(4)&SUBSYS_s(4)n(4)&REV_r(2)
>
> This might be a potential problem for a legacy driver that wants to
> stay vendor-agnostic but I'm pretty sure there would be a reasonable
> way of working around it. Actually, the device must use a designated
> device ID (like 0x1000) in addition to the subsystem device ID so this
> should be a non-issue altogether.
>
For legacy drivers running on modified qemu it can be a problem.
For new virtio-win build we can keep both compatible (VID/DID) together
with full (VID/DID/SID/SVID) specified in .inf file

>> Possibly not only for virtio devices, most pci devices have 1af4:1100
>> as subsystem id, other vendors might want set it too for consistency.
>>
>> cheers,
>>    Gerd