Re: [PATCH RFC 3/5] virtio-pci: extend virtio_pci_cap to hold page_size

[Daniel Verkamp]

On Thu, Feb 13, 2025 at 7:54 AM Sergio Lopez <slp@xxxxxxxxxx> wrote:
>
> Turn the 16 bit padding into a page_size field to allow the device to
> pass its required page size with format PAGE_SIZE >> 12.
>
> Signed-off-by: Sergio Lopez <slp@xxxxxxxxxx>
> ---
>  drivers/virtio/virtio_pci_modern.c | 29 +++++++++++++++++++++++++----
>  include/uapi/linux/virtio_pci.h    |  2 +-
>  2 files changed, 26 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/virtio/virtio_pci_modern.c b/drivers/virtio/virtio_pci_modern.c
> index 79616ce5057bf3b2b88cae7e8fb7729efa9dd632..26e9cd5148c0f10209c34d12e65d64490a855d75 100644
> --- a/drivers/virtio/virtio_pci_modern.c
> +++ b/drivers/virtio/virtio_pci_modern.c
[...]
> +               if (__virtio_test_bit(vdev, VIRTIO_F_SHM_PAGE_SIZE)) {
> +                       pci_read_config_word(dev, pos + offsetof(struct virtio_pci_cap,
> +                                                                page_size), &res_psize);
> +                       if (!res_psize) {
> +                               dev_err(&dev->dev, "%s: shm cap with invalid page size on "
> +                                       "a device with VIRTIO_F_SHM_PAGE_SIZE feature\n",
> +                                       __func__);

Maybe this should also constrain the page size to be a power of 2?

[...]
> diff --git a/include/uapi/linux/virtio_pci.h b/include/uapi/linux/virtio_pci.h
> index 8549d4571257142ac6c9dad5c01369923791a85a..fb0ccb7a125d8178c1f78333c4d2f43540e1764b 100644
> --- a/include/uapi/linux/virtio_pci.h
> +++ b/include/uapi/linux/virtio_pci.h
> @@ -127,7 +127,7 @@ struct virtio_pci_cap {
>         __u8 cfg_type;          /* Identifies the structure. */
>         __u8 bar;               /* Where to find it. */
>         __u8 id;                /* Multiple capabilities of the same type */
> -       __u8 padding[2];        /* Pad to full dword. */
> +       __u16 page_size;        /* Device page size (PAGE_SIZE >> 12). */

This comment should probably clarify that the page_size field is only
valid when cfg_type is VIRTIO_PCI_CAP_SHARED_MEMORY_CFG, assuming
that's the case. Or should the device be expected to provide the
page_size for all capabilities regardless of type?

It seems like the name should also ideally make it clearer that this
is page_size/4096 rather than the actual page size to avoid confusing
device implementers.

>         __le32 offset;          /* Offset within bar. */
>         __le32 length;          /* Length of the structure, in bytes. */
>  };