Re: [virtio-dev] [PATCH 08/11] transport-pci: Introduce virtio extended capability

Parav Pandit
2023-04-10T17:54:20Z

Hi Jason,

On 4/9/2023 9:36 PM, Jason Wang wrote:
> On Fri, Mar 31, 2023 at 7:00 AM Parav Pandit <parav@nvidia.com> wrote:
>>
>> PCI device configuration space for capabilities is limited to only 192
>> bytes shared by many PCI capabilities of generic PCI device and virtio
>> specific.
>>
>> Hence, introduce virtio extended capability that uses PCI Express
>> extended capability.
>> Subsequent patch uses this virtio extended capability.
>>
>> Co-developed-by: Satananda Burla <sburla@marvell.com>
>> Signed-off-by: Parav Pandit <parav@nvidia.com>
>
> Can you explain the differences compared to what I've used to propose?
>
> https://www.mail-archive.com/virtio-dev@lists.oasis-open.org/msg08078.html
>
> This can save time for everybody.
>

What is proposed in this patch similar to [1].

The main difference is, the proposed new capability is always placed in
the pci extended capability section,
This is because legacy capability section is nearly to its full level as
described in commit message.

So providing it at either of the two locations is not valuable.

What you proposed in [1] is in general useful regardless;

However, it is not backward compatible, if the device place them in
extended capability, it will not work.

To make it backward compatible, a device needs to expose existing
structure in legacy area. And extended structure for same capability in
extended pci capability region.

In other words, it will have to be a both places.

Otherwise its similar.

We should do this regardless, and it will also make this series shorter
which is also what Michael prefers.

Would you like join efforts with me of drafting [1] + above description
as independent patch?
We may need it even sooner than this because the AQ patch is expanding
the structure located in legacy area.

[1]
https://www.mail-archive.com/virtio-dev@lists.oasis-open.org/msg08078.html

PASID part of the patch is not relevant here, so will skip to comment.