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

[Pankaj Gupta <pankaj.gupta.linux@gmail.com>]

Hi Stefan,

Thank you for reviewing this.

> > 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.

Yes, Agree.

>
> > +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/

noted.
>
> > +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.

I was not sure we need to add motivation as well in the spec. But yes, will add.
>
> > +
> > +\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

David has answered this.
>
> > +
> > +\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.

But spec is for current implementation?  As we have more devices or ways, we
can amend this.

>
> > +
> > +\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.

Agree.

>
> > +
> > +\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.

Already noted this.
>
> > +
> > +\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.

Sure.

>
> > +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.

Sure.

>
> > +
> > +\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.

noted.
>
> > +
> > +\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?

This will be removed as per earlier feedback.
>
> > +
> > +\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.

o.k

>
> "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.

yes. noted this as per earlier feedback.
>
> > +
> > +\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.

Sure.

>
> > +
> > +\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.

Yes. w
>
> > +
> > +\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).

Sure will do, Thank you!
>
> The same applies to the rest of the security points below.

Yes
>
> > +
> > +\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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