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Subject: Re: Re: Re: [PATCH v2 06/11] transport-fabrics: introduce command set
On Tue, Jun 06, 2023 at 09:31:27AM +0800, zhenwei pi wrote: > On 6/6/23 00:30, Stefan Hajnoczi wrote: > > 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. > > > > OK, thanks. > > > > > > +\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? > > > > I'd like to define two PDU mapping rules(in '[PATCH v2 04/11] > transport-fabrics: introduce Stream Transmission' and '[PATCH v2 05/11] > transport-fabrics: introduce Keyed Transmission'): STREAM and KEYED. A > transport protocols need to use one. > > Then we can define protocols: > #define VIRTIO_OF_CONNECTION_TCP 1 -> use STREAM > #define VIRTIO_OF_CONNECTION_RDMA 2 -> use KEYED > #define VIRTIO_OF_CONNECTION_TLS 3(in the future) -> use STREAM > #define VIRTIO_OF_CONNECTION_XXX It's not clear to me whether TCP actually means TCP/IP or if it actually means STREAM. For example, if I run Virtio Over Fabrics over AF_VSOCK, would it use VIRTIO_OF_CONNECTION_TCP although there is no TCP/IP? If so, then I think the name TCP is misleading and STREAM would be clearer. > > > > > > > > > + 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? > > > > For stream protocol, it always work fine. > For keyed protocol, for example RDMA, the target side needs to use > ibv_post_recv to receive a large size(sizeof virtio_of_command_connect + > sizeof virtio_of_connect). If the target uses ibv_post_recv to receive > sizeof(CMD) + sizeof(DESC) * 1, the initiator fails in RDMA SEND. I read that "A RC connection is very similar to a TCP connection" in the NVIDIA documentation (https://docs.nvidia.com/networking/display/RDMAAwareProgrammingv17/Transport+Modes) and expected SOCK_STREAM semantics for RDMA SEND. Are you saying ibv_post_send() fails when the receiver's work request sg_list size is smaller (fewer bytes) than the sender's? Does the receiver have just 1 WR queued in your example? What happens if the receiver queues multiple small WRs? Stefan
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