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Subject: Re: [virtio-dev] [PATCH RESEND] virtio-pmem: PMEM device spec

On 04.08.21 13:07, Stefan Hajnoczi wrote:
On Wed, Jul 28, 2021 at 05:04:35PM +0200, Pankaj Gupta wrote:
Posting virtio specification for virtio pmem device. Virtio pmem is a
paravirtualized device which allows the guest to bypass page cache.
Virtio pmem kernel driver is merged in Upstream Kernel 5.3. Also, Qemu
device is merged in Qemu 4.1.

Signed-off-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Sorry, It took me long time to get back on this. There is
an enhancement to this spec by "Taylor Stark" CCed in the list.
Request for feedback and merging.

RFC is posted here [1]
[1] https://lists.oasis-open.org/archives/virtio-dev/201903/msg00083.html

I skimmed through the review comments but pretty much reviewed this
patch from scratch. Feel free to ignore questions that others have
already raised.

  conformance.tex |  19 ++++++-
  content.tex     |   1 +
  virtio-pmem.tex | 132 ++++++++++++++++++++++++++++++++++++++++++++++++
  3 files changed, 150 insertions(+), 2 deletions(-)
  create mode 100644 virtio-pmem.tex

diff --git a/conformance.tex b/conformance.tex
index 94d7a06..818ddda 100644
--- a/conformance.tex
+++ b/conformance.tex
@@ -31,7 +31,8 @@ \section{Conformance Targets}\label{sec:Conformance / Conformance Targets}
  \ref{sec:Conformance / Driver Conformance / Sound Driver Conformance},
  \ref{sec:Conformance / Driver Conformance / Memory Driver Conformance},
  \ref{sec:Conformance / Driver Conformance / I2C Adapter Driver Conformance} or
-\ref{sec:Conformance / Driver Conformance / SCMI Driver Conformance}.
+\ref{sec:Conformance / Driver Conformance / SCMI Driver Conformance},
+\ref{sec:Conformance / Driver Conformance / PMEM Driver Conformance}.
\item Clause \ref{sec:Conformance / Legacy Interface: Transitional Device and Transitional Driver Conformance}.
@@ -55,7 +56,8 @@ \section{Conformance Targets}\label{sec:Conformance / Conformance Targets}
  \ref{sec:Conformance / Device Conformance / Sound Device Conformance},
  \ref{sec:Conformance / Device Conformance / Memory Device Conformance},
  \ref{sec:Conformance / Device Conformance / I2C Adapter Device Conformance} or
-\ref{sec:Conformance / Device Conformance / SCMI Device Conformance}.
+\ref{sec:Conformance / Device Conformance / SCMI Device Conformance},
+\ref{sec:Conformance / Device Conformance / PMEM Driver Conformance}.
\item Clause \ref{sec:Conformance / Legacy Interface: Transitional Device and Transitional Driver Conformance}.
@@ -301,6 +303,19 @@ \section{Conformance Targets}\label{sec:Conformance / Conformance Targets}
  \item \ref{drivernormative:Device Types / SCMI Device / Device Operation / Setting Up eventq Buffers}
+\conformance{\subsection}{PMEM Driver Conformance}\label{sec:Conformance / Driver Conformance / PMEM Driver Conformance}
+A PMEM driver MUST conform to the following normative statements:
+\item \ref{devicenormative:Device Types / PMEM Device / Device Initialization}
+\item \ref{drivernormative:Device Types / PMEM Driver / Driver Initialization / Direct access}
+\item \ref{drivernormative:Device Types / PMEM Driver / Driver Initialization / Virtio flush}
+\item \ref{drivernormative:Device Types / PMEM Driver / Driver Operation / Virtqueue command}
+\item \ref{devicenormative:Device Types / PMEM Device / Device Operation / Virtqueue flush}
+\item \ref{devicenormative:Device Types / PMEM Device / Device Operation / Virtqueue return}
  \conformance{\section}{Device Conformance}\label{sec:Conformance / Device Conformance}
A device MUST conform to the following normative statements:
diff --git a/content.tex b/content.tex
index ceb2562..6acc785 100644
--- a/content.tex
+++ b/content.tex
@@ -6583,6 +6583,7 @@ \subsubsection{Legacy Interface: Framing Requirements}\label{sec:Device
\chapter{Reserved Feature Bits}\label{sec:Reserved Feature Bits} diff --git a/virtio-pmem.tex b/virtio-pmem.tex
new file mode 100644
index 0000000..a2b888e
--- /dev/null
+++ b/virtio-pmem.tex
@@ -0,0 +1,132 @@
+\section{PMEM Device}\label{sec:Device Types / PMEM Device}
+The virtio pmem is a fake persistent memory (NVDIMM) device

s/fake/virtual/ or drop "fake"? If the device persists data correctly
then it's not fake.

+used to bypass the guest page cache and provide a virtio
+based asynchronous flush mechanism.This avoids the need
+of a separate page cache in guest and keeps page cache only
+in the host. Under memory pressure, the host makes use of
+effecient memory reclaim decisions for page cache pages


+of all the guests. This helps to reduce the memory footprint
+and fit more guests in the host system.

This explains the motivation for the device. It would also be nice to
explain the nature of the device:

   The virtio pmem device provides access to byte-addressable persistent
   memory. The persist memory is directly accessible as a Shared Memory
   Region. 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}
+\item[0] req_vq
+\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}
+struct virtio_pmem_config {
+	le64 start;
+	le64 size;
+\item[\field{start}] contains the start address from the guest physical address range
+to be hotplugged into the guest address space using the pmem API.
+\item[\field{size}] contains the length of this address range.

Please use a Shared Memory Region instead. This is a relatively new
addition to the VIRTIO device model that seems like a good fit for this

Note that virtio-pmem is already upstream in QEMU and in Linux using this model. There is a proposal to optionally use shared memory for exposing the area, to be unlocked with a feature bit.


David / dhildenb

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