Re: [PATCH v1 1/2] transport-pci: Introduce legacy registers access commands

[Parav Pandit <parav@nvidia.com>]

On 5/3/2023 12:49 PM, Michael S. Tsirkin wrote:
> On Wed, May 03, 2023 at 10:49:14AM -0400, Parav Pandit wrote:
> > On 5/3/2023 1:50 AM, Michael S. Tsirkin wrote:
> > > On Wed, May 03, 2023 at 06:26:58AM +0300, Parav Pandit wrote:
> >
> > > > +Legacy registers write admin command follows \field{struct virtio_admin_cmd}.
> > > > +This command writes legacy registers of a member VF device. Driver should write
> > > > +appropriate register \field{size} depending on the width of the legacy
> > > > +common registers or device specific registers.
> > > > +Driver sets command \field{opcode} to VIRTIO_ADMIN_CMD_LREG_WRITE.
> > > > +Driver sets \field{group_type} to 1 for VFs.
> > > > +Driver sets \field{group_member_id} to a valid VF number.
> > > > +
> > > > +The \field{command_specific_data} has following listed structure format:
> > > > +
> > > > +\begin{lstlisting}
> > > > +struct virtio_admin_cmd_lreg_wr_data {
> > > > +	u8 offset; /* Starting byte offset of the register(s) to write */
> > > > +	u8 size; /* Number of bytes to write into the register. */
> > > > +	u8 register[];
> > >
> > > And maybe add
> > > 	u8 reserved[]; /* structure padding to multiple of 8 bytes */
> > >
> > > > +};
> > Thinking, what do we miss without the padding here?
>
> it's not 100% clear where the padding is.
It is same as rest of the other virtio structures as described in 
"Structure Specifications" section.

"Many device and driver in-memory structure layouts are documented using 
the C struct syntax. All structures
are assumed to be without additional padding."