RE: [virtio-comment] RE: [PATCH v8 8/9] admin: command list discovery

["Uminski, Piotr" <Piotr.Uminski@intel.com>]

> From: Michael S. Tsirkin <mst@redhat.com>
> 
> On Mon, Nov 21, 2022 at 11:06:14AM +0000, Uminski, Piotr wrote:
> > Instead of a bit-coded list of supported commands, can we just use an
> array of supported command values? It allows us to use non-contiguous
> command coding and checking how big  is an array:       le32
> device_admin_cmds[];
> 
> Interesting, what's the usecase exactly?
In some protocols command IDs are grouped, for example:
	Commands 0-99: standard device management
	Commands 100 - 199: vendor-specific device management
	Commands 200 - 299: standard queue management
	Commands 300 - 399: vendor-specific queue management
Initially in each group only some command IDs are used. It makes it easier to add new commands later.
> I don't really expect more than order or 200 commands - about 8 dwords.
Agree that the number of commands will not be big. However if we define a command #1 (say standard one) and command #100 (say vendor-specific), we need to use 4 dwords for 2 commands anyway. 

> And the operation we normally need is checking whether a given command
> is supported, with a list that will be a slow lookup.
I don't think that is a significant difference - this is once-per-initialization operation.

> Besides with your idea driver first needs to check how many commands are
> supported, then do the query - two roundtrips to device.
Right - this is the main disadvantage of my proposal.

> With the proposed interface, driver just allocates a buffer for commands it
> knows about. Device truncates the buffer to the length supplied by driver, so
> any commands outside the buffer are ignored.
> 
> --
> MST

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