Re: [PATCH v2 1/4] Add virtio Admin Virtqueue

[Cornelia Huck <cohuck@redhat.com>]

On Mon, Jan 31 2022, "Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Mon, Jan 31, 2022 at 03:26:36PM +0100, Cornelia Huck wrote:
>> On Mon, Jan 31 2022, "Michael S. Tsirkin" <mst@redhat.com> wrote:
>> 
>> > On Mon, Jan 31, 2022 at 10:16:43AM +0100, Cornelia Huck wrote:
>> >> On Sun, Jan 30 2022, "Michael S. Tsirkin" <mst@redhat.com> wrote:
>> >> 
>> >> > On Sun, Jan 30, 2022 at 05:12:46PM +0200, Max Gurtovoy wrote:
>> >> >> #define VIRTIO_PCI_CAP_MISC_CFG 10
>> >> >> 
>> >> >> and
>> >> >> 
>> >> >> struct virtio_pci_misc_cfg {
>> >> >>     le16 admin_queue_index; /* read-only for driver */
>> >> >> };
>> >> >> 
>> >> >> Is agreed by all for V3 ? instead of the net and blk AQ index definitions.
>> >> >
>> >> > We need to add it to MMIO and CCW I guess too.
>> >> 
>> >> That seems ok for pci.
>> >> 
>> >> For ccw, I'd do something like
>> >> 
>> >> #define CCW_CMD_READ_MISC_CONF 0x82
>> >> 
>> >> struct virtio_misc_conf {
>> >>        be16 admin_queue_index;
>> >> };
>> >> 
>> >> bound to revision 3, which gets a payload data containing the length of
>> >> this structure (for future expansions).
>> >> 
>> >> Halil, do you think that would work?
>> >> 
>> >> For mmio, I'd need to think a bit more. Any mmio experts around?
>> >
>> > Not an expert but I think we can rely on a feature
>> > bit to be acked since admin vq is only needed
>> > after feature negotiation is complete.
>> 
>> You mean a register that is valid conditionally? I don't see an easy way
>> to add some kind of "misc" interface for mmio, unlike for the other
>> transports.
>> 
>> So something like:
>> 
>> AdminQueueIndex/0x0c4/R
>> If VIRTIO_F_ADMIN_VQ has been negotiated, reading from this register
>> returns the queue index of the administration virtqueue.
>
> No, I mean a register that switches 100+ between device specific
> and misc space.

Do you want to make that dependent on an extra feature bit, then? We
certainly want to make that facility available if the admin vq is not
negotiated but another future feature using misc is. (We probably don't
want to bump the device version.)