[virtio-comment] Re: Re: [PATCH v2 06/11] transport-fabrics: introduce command set

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Jun 02, 2023 at 01:15:00PM +0800, zhenwei pi wrote:
> 
> 
> On 6/1/23 01:10, Stefan Hajnoczi wrote:
> > On Thu, May 04, 2023 at 04:19:05PM +0800, zhenwei pi wrote:
> > > Introduce command structures for Virtio-oF.
> > > 
> > > Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>
> > > ---
> > >   transport-fabrics.tex | 209 ++++++++++++++++++++++++++++++++++++++++++
> > >   1 file changed, 209 insertions(+)
> > > 
> > > diff --git a/transport-fabrics.tex b/transport-fabrics.tex
> > > index 7711321..37f57c6 100644
> > > --- a/transport-fabrics.tex
> > > +++ b/transport-fabrics.tex
> > > @@ -495,3 +495,212 @@ \subsubsection{Buffer Mapping Definition}\label{sec:Virtio Transport Options / V
> > >                       |value |  -> 8193 (value.u32)
> > >                       +------+
> > >   \end{lstlisting}
> > > +
> > > +\subsubsection{Commands Definition}\label{sec:Virtio Transport Options / Virtio Over Fabrics / Transmission Protocol / Commands Definition}
> > > +This section defines command structures for Virtio Over Fabrics.
> > > +
> > > +A common structure virtio_of_value is fixed to 8 bytes and MUST be used as one
> > > +of the following format:
> > > +
> > > +\begin{itemize}
> > > +\item u8
> > > +\item le16
> > > +\item le32
> > > +\item le64
> > > +\end{itemize}
> > 
> > The way it's written does not document where the u8, u16, u32 bytes are
> > located and that the unused bytes are 0. I think I understand what you
> > mean though:
> > 
> >    le64 value = cpu_to_le64((u64)v); /* v is u8, u16, u32, or u64 */
> > 
> > Please clarify.
> > 
> 
> I want to describe an union structure of 8 bytes:
> union virtio_of_value {
>     u8;
>     u16;
>     u32;
>     u64;
> };
> 
> Depending on the opcode, use the right one.

I was trying to point out that the memory layout of C unions is not
portable. Your example does not define the exact in-memory layout of
union virtio_of_value. Here is the first web search result I found about
this topic:

  "Q: And a related question: if you dump unions in binary form to a file,
  and then reload them from the file on a different platform, or with a
  program compiled by a different compiler, are you guaranteed to get
  back what you stored? (I think not, but I'm not sure)

  A: You're right; you're not."

  https://bytes.com/topic/c/answers/220372-unions-storage-abis

In the cpu_to_le64() code example that I gave, the exact in-memory
layout is well-defined. There is no ambiguity.

> > > +\hline
> > > +0xff00 - 0xfffd & Reserved \\
> > > +\hline
> > > +\end{tabular}
> > > +
> > > +\paragraph{Connect Command}\label{sec:Virtio Transport Options / Virtio Over Fabrics / Transmission Protocol / Commands Definition / Connect Command}
> > > +The Connect Command is used to establish Virtio Over Fabrics queue. The control
> > > +queue MUST be established firstly, then the Connect command establishes an
> > > +association between the initiator and the target.
> > 
> > Is a "Virtio Over Fabrics queue" different from a virtqueue?
> > 
> > If I understand correctly, the control queue must be established by the
> > initiator first and then the Connect command is sent to begin
> > communication between the initiator and the target?
> > 
> 
> The queue mapping is missing in the '[PATCH v2 01/11] transport-fabrics:
> introduce Virtio Over Fabrics overview', like:
> A "Virtio Over Fabrics queue" is a reliable connection between initiator and
> target. There are 2 types of Virtio Over Fabrics queue:
> +\begin{itemize}
> +\item A single Control queue is required to execute control operations.
> +\item 0 or more Virtio Over Fabrics queues map the virtqueues.
> +\end{itemize}

That helps, thanks!

> 
> > > +
> > > +The Target ID of 0xffff is reserved, then:
> > 
> > Please move this after the fields have been shown and the purpose of the
> > Target ID field has been explained.
> > 
> > > +\begin{itemize}
> > > +\item The Target ID of 0xffff MUST be specified as the Target ID in a Connect
> > > +Command for the control queue.
> > > +\item The target SHOULD allocate any available Target ID to the initiator,
> > > +and return the allocated Target ID in the Completion.
> > > +\item The returned Target ID MUST be specified as the Target ID, and the Queue ID
> > > +MUST be specified in a Connect Command for the virtqueue.
> > > +\end{itemize}
> > 
> > What is the purpose of the Target ID? Is it to allow a server to provide
> > access to multiple targets over the same connection?
> > 
> 
> A target listens on a port, and provides access to 0 or more targets. An
> initiator connect the specific target by TVQN of connect command.
> An initiator could connect a single target, multiple initiators could
> connect the same target(typically, shared disk/fs).

Why is the target ID separate from the TVQN? If the Target ID is a
separate parameter then users will have to learn additional
syntax/command-line options to specify the TVQN + Target ID and that
syntax may vary between software.

> 
> > > +
> > > +The Connect Command has following structure:
> > > +
> > > +\begin{lstlisting}
> > > +struct virtio_of_command_connect {
> > > +        le16 opcode;
> > > +        le16 command_id;
> > > +        le16 target_id;
> > > +        le16 queue_id;
> > > +        le16 ndesc;
> > 
> > Where is this field documented?
> > 
> 
> OK. Will add.
> 
> > Why does the initiator send ndesc to the target? Normally a VIRTIO Transport reports the device's max descriptors and then the driver can tell the device to reduce the number of descriptors, if desired.
> > 
> 
> A target supports at lease 1 descriptor. The 'ndesc' of struct
> virtio_of_command_connect indicates the full PDU contains: struct
> virtio_of_command_connect + 1 * virtio_of_vq_desc + data.
> 
> > > +#define VIRTIO_OF_CONNECTION_TCP     1
> > > +#define VIRTIO_OF_CONNECTION_RDMA    2
> > 
> > What does RDMA mean? I thought RDMA is a general concept that several
> > fabrics implement (with different details like how addressing works).
> > 
> 
> I guest your concern is the difference of IB/RoCE/iWarp ...
> We are trying to define the payload protocol here, so I think we can ignore
> the difference of the HCA.

I see, maybe this could be called STREAM vs KEYED instead of TCP vs RDMA?

> 
> > > +        u8 oftype;
> > > +        u8 padding[5];
> > > +};
> > > +\end{lstlisting}
> > > +
> > > +The Connect commands MUST contains one Segment Descriptor and one structure
> > > +virtio_of_command_connect to specify Initiator VQN and Target VNQ,
> > > +virtio_of_command_connect has following structure:
> > 
> > I'm confsued. virtio_of_command_connect was defined above. The struct
> > defined below is virtio_of_connect. Does this paragraph need to be
> > updated (virtio_of_command_connect -> virtio_of_connect)?
> > 
> > Why is virtio_of_connect a separate struct and not part of
> > virtio_of_command_connect?
> > 
> 
> Because I'd like to define all the commands with a fixed length.

I don't understand. virtio_of_connect and virtio_of_command_connect are
both fixed-length. Why can't they be unified into 1 fixed-length struct?

> > It's not possible to review this patch because these structs aren't used
> > yet and the opcodes are undefined.
> > 
> > Defining structs that are shared by multiple opcodes might make
> > implementations cleaner, but I think it makes the spec less clear. I
> > would rather have a list of all opcodes and each one shows the full
> > command layout (even if it is duplicated). That way it's very easy to
> > look up an opcode you are implementing or debugging and check what's
> > needed. If the command layout is not documented in a single place, then
> > it takes more effort to figure out how an opcode works.
> > 
> > Stefan
> 
> OK, I'll merge the structure definition into the opcode definition.

Thank you!

Stefan

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