Re: [PATCH v10 4/4] transport-pci: Introduce group legacy group member config region access

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jul 06, 2023 at 04:58:51PM +0000, Parav Pandit wrote:
> 
> 
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Thursday, July 6, 2023 12:42 PM
> > 
> > We will need to detail how notifications work though. The reason is that the
> > legacy notifications are received by owner driver (which is not legacy!) and
> > forwarded to device.
> > We need to explain exactly how this is done for people who don't read minds
> > and did not follow 3 months of mailing list debates, and explain how this has the
> > same effect as triggering driver notification through the legacy interface.
> >
> I am not going to write driver implementation in the spec here.
> But I agree some high-level text is needed.
> With your suggestions
> 1. to merge pci and generic in one section,
> 2. add description to notification,
> 3. past rewrites provided in v9,
> 
> The draft of v11, looks like below. Please see how much more verbose to make it, if at all.
> 
> \paragraph{Legacy Driver Notification}\label{par:Basic Facilities of a Virtio Device / Device groups / Group administration commands / Legacy Interface / Legacy Driver Notifications}
> 
> If the group owner device or the group member device support driver
> notifications via a memory-mapped operation or I/O operation, these
> notifications are sent to the device via accessing such a notification region
> using a memory or I/O operation instead of sending the notifications through
> the administration command.

This paragraph adds very little information - just some vague hints at how things work.

> The driver of the owner device can send a driver notification to the member
> device by executing VIRTIO_ADMIN_CMD_LEGACY_COMMON_CFG_WRITE with the
> \field{offset} matching \field{Queue Notify} and the \field{data} containing
> the virtqueue index to be notified.
> 
> However, as many administration commands are used for slow path configuration,
> a separate fast path mechanism for such notifications is desired. For the
> SR-IOV group type, the optional command VIRTIO_ADMIN_CMD_LEGACY_NOTIFY_INFO
> addresses this need by returning to the driver one or more addresses which to
> be used to send such driver notifications.
> 
> In one example, the driver in the hypervisor which intercepts I/O BAR accesses
> for the \field{Queue Notify} can access the notification area directly as memory
> or I/O access (depending on what the device reported) instead of slow
> administration command.
> 
> For the command VIRTIO_ADMIN_CMD_LEGACY_NOTIFY_INFO the driver sets
> \field{opcode} to 0x6.
> The driver sets \field{group_member_id} which refers to the member device to be
> accessed. This command does not use \field{command_specific_data}.
> 
> This command is currently only defined for the SR-IOV group type. When the
> device supports VIRTIO_ADMIN_CMD_LEGACY_NOTIFY_INFO command, the group owner
> device hardwires VF BAR0 to zero in the SR-IOV Extended capability and the
> group member device does not use PCI BAR0 in all the Virtio PCI capabilities
> listed in section \ref{sec:Virtio Transport Options / Virtio Over PCI Bus / Virtio Structure PCI Capabilities}.

After reading this I still have no clue at how these notifications work.
maybe it's in some other patch. Let's see v11.

Just making a wild guess:

it is true that
	to supply a driver notification to the device, it is possible
for the driver to write the vq index of the vq to be notified, in little
endian format, using a single 16 byte write at the address provided ?

and I am guessing that
	the write is an IO or a memory write, depending on the
	type of the BAR referred to?

then why don't we just say this?


-- 
MST