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


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}.
>    \end{itemize}
> @@ -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}.
>    \end{itemize}
> @@ -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}
>  \end{itemize}
>  
> +\conformance{\subsection}{PMEM Driver Conformance}\label{sec:Conformance / Driver Conformance / PMEM Driver Conformance}
> +
> +A PMEM driver MUST conform to the following normative statements:
> +
> +\begin{itemize}
> +\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}
> +\end{itemize}
> +
>  \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
>  \input{virtio-mem.tex}
>  \input{virtio-i2c.tex}
>  \input{virtio-scmi.tex}
> +\input{virtio-pmem.tex}
>  
>  \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

s/effecient/efficient/

> +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}
> +\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 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.
> +\end{description}

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
device:
https://github.com/oasis-tcs/virtio-spec/blob/master/shared-mem.tex

> +
> +\subsection{Device Initialization}\label{sec:Device Types / PMEM Device / Device Initialization}
> +
> +Device hotplugs physical memory to guest address space. Persistent memory device
> +is emulated with file backed memory at host side.

This paragraph can be dropped. The device could be implemented in a
number of ways and it's beyond the scope of the specification.

> +
> +\begin{enumerate}
> +\item Guest vpmem start is read from \field{start}.
> +\item Guest vpmem end is read from \field{size}.
> +\end{enumerate}

Please avoid the terms "host" and "guest". VIRTIO devices can also be
implemented in hardware and used without guests. The spec usually uses
the terms "device" and "driver" instead.

> +
> +\devicenormative{\subsubsection}{Device Initialization}{Device Types / PMEM Device / Device Initialization}
> +
> +File backed memory MUST be memory mapped to guest address space with SHARED
> +memory mapping.

This is a device implementation detail that is beyond the scope of the
specification. The VIRTIO spec is concerned with the driver/device
interface, not with the implementation details of the device.

> +
> +\subsection{Driver Initialization}\label{sec:Device Types / PMEM Driver / Driver Initialization}
> +
> +Driver hotplugs the physical memory and registers associated region with the pmem API.

The pmem API is a Linux kernel implementation detail. Instead you could
say something like:

  The driver determines the start address and size of the persist memory
  region in preparation for reading or writing data.

> +Also, configures a flush callback function with the corresponding region.

"callback function" is a driver implementation detail that's beyond the
scope of the specification. Instead you could say something like:

  The driver initializes req_vq in preparation for making flush
  requests.

> +
> +\drivernormative{\subsubsection}{Driver Initialization: Filesystem direct access}{Device Types / PMEM Driver / Driver Initialization / Direct access}
> +
> +Driver MUST enable filesystem direct access operations for read/write on the device.

This is beyond the scope of the VIRTIO specification because it's a
Linux guest kernel detail.

> +
> +\drivernormative{\subsubsection}{Driver Initialization: Virtio flush}{Device Types / PMEM Driver / Driver Initialization / Virtio flush}
> +
> +Driver MUST implement a virtio based flush callback.

Driver implementation detail.

> +
> +Driver MUST disable other FLUSH/SYNC mechanisms for the device when virtio flush is configured.

What does this mean?

> +
> +\subsection{Driver Operations}\label{sec:Device Types / PMEM Driver / Driver Operation}
> +\drivernormative{\subsubsection}{Driver Operation: Virtqueue command}{Device Types / PMEM Driver / Driver Operation / Virtqueue command}
> +
> +Driver MUST send VIRTIO_FLUSH command on request virtqueue, allows guest userspace process to perform IO operations asynchronously.

VIRTIO_FLUSH has not been defined.

"guest userspace process" is beyond the scope of the driver/device
interface and therefore not relevant to the VIRTIO specification. This
could be rephrased:

  The driver MUST send and wait for the successful completion of a
  VIRTIO_PMEM_FLUSH command on req_vq in order to ensure previously
  written data will persist across device reset and power failure.

> +
> +Driver SHOULD handle multiple fsync requests on files present on the device.

The concept of "files" is beyond the scope of the driver/device
interface. We only deal with the memory region in this specification, so
I think the scenario you're describing is when multiple writes have been
performed and there are several flush commands in flight.

> +
> +\subsection{Device Operations}\label{sec:Device Types / PMEM Driver / Device Operation}
> +
> +\devicenormative{\subsubsection}{Device Operations}{Device Types / PMEM Device / Device Operation / Virtqueue flush}
> +
> +Device SHOULD handle multiple flush requests simultaneously using host filesystem fsync or flush call.

Same thing as above.

Missing:

  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.

> +
> +\devicenormative{\subsubsection}{Device operations}{Device Types / PMEM Device / Device Operation / Virtqueue return}
> +
> +Device MUST return integer "0" for success and "-1" for failure.
> +These errors are converted to corresponding error codes by guest
> +as per architecture.

This sentence about guest error codes is outside the scope of the VIRTIO
specification.

> +
> +\subsection{Possible security implications}\label{sec:Device Types / PMEM Device / Possible Security Implications}
> +
> +There could be potential security implications depending on how
> +memory mapped host backing file is used. By default device emulation
> +is done with SHARED mapping. There is a contract between guest and host
> +process to access same backing file for read/write operations.
> +
> +If a malicious guest or host userspace map the same backing file,
> +attacking process can make use of known cache side channel attacks
> +to predict the current state of shared page cache page. If both
> +attacker and victim somehow execute same shared code after a
> +flush/evict call, with difference in execution timing attacker
> +could infer another guest local data or host data. Though this is
> +not easy and same challenges exist as with bare metal host system
> +when userspace share same backing file.

This is important information but needs to be phrased without referring
to "host"/"host" and only with respect to the driver/device interface
(not the host/guest kernel, applications, etc).

The same applies to the rest of the security points below.

> +
> +\subsection{Countermeasures}\label{sec:Device Types / PMEM Device / Possible Security Implications / Countermeasures}
> +
> +\subsubsection{ With SHARED mapping}\label{sec:Device Types / PMEM Device / Possible Security Implications / Countermeasures / SHARED}
> +
> +If device backing backing file is shared with multiple guests or host
> +processes, this may act as a metric for page cache side channel attack.
> +As a counter measure every guest should have its own(not shared with
> +another guest) SHARED backing file and gets populated a per host process
> +page cache pages.
> +
> +\subsubsection{ With PRIVATE mapping}\label{sec:Device Types / PMEM Device / Possible Security Implications / Countermeasures / PRIVATE}
> +There maybe be chances of side channels attack with PRIVATE
> +memory mapping similar to SHARED with read-only shared mappings.
> +PRIVATE is not used for virtio pmem making this usecase
> +irrelevant.
> +
> +\subsubsection{ Workload specific mapping}\label{sec:Device Types / PMEM Device / Possible Security Implications / Countermeasures / Workload}
> +For SHARED mapping, if workload is single application inside
> +guest and there is no risk with sharing of data between guests.
> +Guest sharing same backing file with SHARED mapping can be
> +used as a valid configuration.
> +
> +\subsubsection{ Prevent cache eviction}\label{sec:Device Types / PMEM Device / Possible Security Implications / Countermeasures / Cache eviction}
> +Don't allow cache evict from guest filesystem trim/discard command
> +with virtio pmem. This rules out any possibility of evict-reload
> +page cache side channel attacks if backing disk is shared(SHARED)
> +with mutliple guests. Though if we use per device backing file with
> +shared mapping this countermeasure is not required.
> -- 
> 2.25.1
> 
> 
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