Re: [PATCH v3] virtio-pmem: PMEM device spec

[Stefan Hajnoczi <stefanha@redhat.com>]

On Thu, Sep 02, 2021 at 07:40:33AM +0200, Pankaj Gupta wrote:
> diff --git a/virtio-pmem.tex b/virtio-pmem.tex
> new file mode 100644
> index 0000000..6f1b504
> --- /dev/null
> +++ b/virtio-pmem.tex
> @@ -0,0 +1,122 @@
> +\section{PMEM Device}\label{sec:Device Types / PMEM Device}
> +
> +The virtio pmem device is a persistent memory (NVDIMM) device
> +that provides a virtio based asynchronous flush mechanism. This avoids
> +the need of a separate page cache in the guest and keeps the page cache
> +only in the host. Under memory pressure, the host makes use of
> +efficient memory reclaim decisions for page cache pages of all the
> +guests. This helps to reduce the memory footprint and fits more guests
> +in the host system.
> +
> +The virtio pmem device provides access to byte-addressable persistent
> +memory. The persist memory is directly accessible range of system memory.

s/is directly accessible range/is a directly accessible range/

> +Data written to this memory is made persistent by separately sending a
> +flush command. Writes that have been flushed are preserved across device
> +reset and power failure.
> +
> +\subsection{Device ID}\label{sec:Device Types / PMEM Device / Device ID}
> +  27
> +
> +\subsection{Virtqueues}\label{sec:Device Types / PMEM Device / Virtqueues}
> +\begin{description}
> +\item[0] req_vq
> +\end{description}
> +
> +\subsection{Feature bits}\label{sec:Device Types / PMEM Device / Feature bits}
> +
> +There are currently no feature bits defined for this device.
> +
> +\subsection{Device configuration layout}\label{sec:Device Types / PMEM Device / Device configuration layout}
> +
> +\begin{lstlisting}
> +struct virtio_pmem_config {
> +	le64 start;
> +	le64 size;
> +};
> +\end{lstlisting}
> +
> +\begin{description}
> +\item[\field{start}] contains the physical address of the first byte of the persistent memory region.
> +
> +\item[\field{size}] contains the length of this address range.
> +\end{description}
> +
> +\begin{enumerate}
> +\item Driver vpmem start is read from \field{start}.
> +\item Driver vpmem end is read from \field{size}.
> +\end{enumerate}
> +
> +\subsection{Driver Initialization}\label{sec:Device Types / PMEM Driver / Driver Initialization}
> +
> +The driver determines the start address and size of the persistent memory region in preparation for reading or writing data.
> +
> +The driver initializes req_vq in preparation for making flush requests.
> +
> +\subsection{Driver Operations}\label{sec:Device Types / PMEM Driver / Driver Operation / Request Queues}
> +
> +Requests have the following format:
> +
> +\begin{lstlisting}
> +struct virtio_pmem_req {
> +        le32 type;
> +};
> +\end{lstlisting}
> +
> +\field{type} is the request command type. 

Where are the type constants defined? I was expecting a flush request
type constant:

  /* Request types */
  #define VIRTIO_PMEM_REQ_T_FLUSH 0

> +
> +\subsection{Device Operations}\label{sec:Device Types / PMEM Driver / Device Operation}
> +\devicenormative{\subsubsection}{Device Operation: Virtqueue flush}{Device Types / PMEM Device / Device Operation / Virtqueue flush}
> +
> +The device MUST ensure that all writes made before a flush request will persist across device reset and power failure before completing the flush request.

"made" is vague, does it mean "submitted" or "completed". I suggest
tweaking the language:

"writes completed before a flush request persist ..."

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