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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 > > > --------------------------------------------------------------------- > To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org > For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org >
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