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Subject: Re: [virtio-dev] [PATCH v3] add virtio-pmem device specification
Ping.
Cornelia, reviewed the spec except pmem specific changes.
Incorporated all the review suggestions.
Thanks,
Pankaj
>
> This patch proposes a virtio specification for new
> virtio pmem device. Virtio pmem is a paravirtualized
> device which solves two problems:
>
> - Provides emulation of persistent memory on host regular
> (non NVDIMM) storage.
> - Allows the guest to bypass the page cache.
>
> Signed-off-by: Pankaj Gupta <pagupta@redhat.com>
> ---
> This is changed version from previous v2 [1], as per suggestions by
> cornelia. Incorporated v2 changes suggested by Stefan, Michael &
> Cornerlia in RFC[2].
>
> [1] https://lists.oasis-open.org/archives/virtio-dev/201907/msg00007.html
> [2] https://lists.oasis-open.org/archives/virtio-dev/201903/msg00083.html
>
> conformance.tex | 22 ++++++++++--
> content.tex | 1 +
> virtio-pmem.tex | 109
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 130 insertions(+), 2 deletions(-)
> create mode 100644 virtio-pmem.tex
>
> diff --git a/conformance.tex b/conformance.tex
> index 42f702a..b383ef3 100644
> --- a/conformance.tex
> +++ b/conformance.tex
> @@ -15,14 +15,14 @@ \section{Conformance Targets}\label{sec:Conformance /
> Conformance Targets}
> \begin{itemize}
> \item Clause \ref{sec:Conformance / Driver Conformance}.
> \item One of clauses \ref{sec:Conformance / Driver Conformance / PCI
> Driver Conformance}, \ref{sec:Conformance / Driver Conformance / MMIO
> Driver Conformance} or \ref{sec:Conformance / Driver Conformance /
> Channel I/O Driver Conformance}.
> - \item One of clauses \ref{sec:Conformance / Driver Conformance / Network
> Driver Conformance}, \ref{sec:Conformance / Driver Conformance / Block
> Driver Conformance}, \ref{sec:Conformance / Driver Conformance / Console
> Driver Conformance}, \ref{sec:Conformance / Driver Conformance / Entropy
> Driver Conformance}, \ref{sec:Conformance / Driver Conformance / Traditional
> Memory Balloon Driver Conformance}, \ref{sec:Conformance / Driver
> Conformance / SCSI Host Driver Conformance}, \ref{sec:Conformance / Driver
> Conformance / Input Driver Conformance}, \ref{sec:Conformance / Driver
> Conformance / Crypto Driver Conformance} or \ref{sec:Conformance / Driver
> Conformance / Socket Driver Conformance}.
> + \item One of clauses \ref{sec:Conformance / Driver Conformance / Network
> Driver Conformance}, \ref{sec:Conformance / Driver Conformance / Block
> Driver Conformance}, \ref{sec:Conformance / Driver Conformance / Console
> Driver Conformance}, \ref{sec:Conformance / Driver Conformance / Entropy
> Driver Conformance}, \ref{sec:Conformance / Driver Conformance / Traditional
> Memory Balloon Driver Conformance}, \ref{sec:Conformance / Driver
> Conformance / SCSI Host Driver Conformance}, \ref{sec:Conformance / Driver
> Conformance / Input Driver Conformance}, \ref{sec:Conformance / Driver
> Conformance / Crypto Driver Conformance}, \ref{sec:Conformance / Driver
> Conformance / Socket Driver Conformance} or \ref{sec:Conformance / Driver
> Conformance / PMEM Driver Conformance}.
> \item Clause \ref{sec:Conformance / Legacy Interface: Transitional
> Device and Transitional Driver Conformance}.
> \end{itemize}
> \item[Device] A device MUST conform to four conformance clauses:
> \begin{itemize}
> \item Clause \ref{sec:Conformance / Device Conformance}.
> \item One of clauses \ref{sec:Conformance / Device Conformance / PCI
> Device Conformance}, \ref{sec:Conformance / Device Conformance / MMIO
> Device Conformance} or \ref{sec:Conformance / Device Conformance /
> Channel I/O Device Conformance}.
> - \item One of clauses \ref{sec:Conformance / Device Conformance / Network
> Device Conformance}, \ref{sec:Conformance / Device Conformance / Block
> Device Conformance}, \ref{sec:Conformance / Device Conformance / Console
> Device Conformance}, \ref{sec:Conformance / Device Conformance / Entropy
> Device Conformance}, \ref{sec:Conformance / Device Conformance / Traditional
> Memory Balloon Device Conformance}, \ref{sec:Conformance / Device
> Conformance / SCSI Host Device Conformance}, \ref{sec:Conformance / Device
> Conformance / Input Device Conformance}, \ref{sec:Conformance / Device
> Conformance / Crypto Device Conformance} or \ref{sec:Conformance / Device
> Conformance / Socket Device Conformance}.
> + \item One of clauses \ref{sec:Conformance / Device Conformance / Network
> Device Conformance}, \ref{sec:Conformance / Device Conformance / Block
> Device Conformance}, \ref{sec:Conformance / Device Conformance / Console
> Device Conformance}, \ref{sec:Conformance / Device Conformance / Entropy
> Device Conformance}, \ref{sec:Conformance / Device Conformance / Traditional
> Memory Balloon Device Conformance}, \ref{sec:Conformance / Device
> Conformance / SCSI Host Device Conformance}, \ref{sec:Conformance / Device
> Conformance / Input Device Conformance}, \ref{sec:Conformance / Device
> Conformance / Crypto Device Conformance}, \ref{sec:Conformance / Device
> Conformance / Socket Device Conformance} or \ref{sec:Conformance / Device
> Conformance / PMEM Device Conformance}.
> \item Clause \ref{sec:Conformance / Legacy Interface: Transitional
> Device and Transitional Driver Conformance}.
> \end{itemize}
> \end{description}
> @@ -183,6 +183,14 @@ \section{Conformance Targets}\label{sec:Conformance /
> Conformance Targets}
> \item \ref{drivernormative:Device Types / Socket Device / Device Operation /
> Device Events}
> \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{drivernormative:Device Types / PMEM Driver / Driver Operation /
> Virtqueue command}
> +\end{itemize}
> +
> \conformance{\section}{Device Conformance}\label{sec:Conformance / Device
> Conformance}
>
> A device MUST conform to the following normative statements:
> @@ -336,6 +344,16 @@ \section{Conformance Targets}\label{sec:Conformance /
> Conformance Targets}
> \item \ref{devicenormative:Device Types / Socket Device / Device Operation /
> Receive and Transmit}
> \end{itemize}
>
> +\conformance{\subsection}{PMEM Device Conformance}\label{sec:Conformance /
> Device Conformance / PMEM Device Conformance}
> +
> +A PMEM device MUST conform to the following normative statements:
> +
> +\begin{itemize}
> +\item \ref{devicenormative:Device Types / PMEM Device / Device
> Initialization}
> +\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}{Legacy Interface: Transitional Device and
> Transitional Driver Conformance}\label{sec:Conformance / Legacy Interface:
> Transitional Device and Transitional Driver Conformance}
> A conformant implementation MUST be either transitional or
> non-transitional, see \ref{intro:Legacy
> diff --git a/content.tex b/content.tex
> index 8f0498e..28e747c 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -5598,6 +5598,7 @@ \subsubsection{Legacy Interface: Framing
> Requirements}\label{sec:Device
> \input{virtio-input.tex}
> \input{virtio-crypto.tex}
> \input{virtio-vsock.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..b824ffe
> --- /dev/null
> +++ b/virtio-pmem.tex
> @@ -0,0 +1,109 @@
> +\section{PMEM Device}\label{sec:Device Types / PMEM Device}
> +
> +virtio pmem is an emulated persistent memory device using virtio.
> +
> +The device works as fake nvdimm device when emulated on a host regular
> +(non NVDIMM) device. The device provides a virtio based asynchronous
> +flush mechanism to persist the guest writes. This avoids the
> +need of separate caching inside the guest and host side caching
> +is used. Under memory pressure, the host makes efficient memory
> +reclaim decisions on uniform view of memory.
> +
> +\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 {
> + uint64_t start;
> + uint64_t size;
> +};
> +\end{lstlisting}
> +
> +\field{start} contains the physical address of the start of the persistent
> memory range.
> +\field{size} contains the length of address range in bytes.
> +
> +\subsection{Device Initialization}\label{sec:Device Types / PMEM Device /
> Device Initialization}
> +
> +The device hot-plugs physical memory to guest address space. The persistent
> memory device
> +is emulated at host side.
> +
> +\begin{enumerate}
> + \item The driver reads the physical start address from \field{start}.
> + \item The driver reads the length of the persistent memory range from
> \field{size}.
> + \end{enumerate}
> +
> +\devicenormative{\subsubsection}{Device Initialization}{Device Types / PMEM
> Device / Device Initialization}
> +
> +The host memory region MUST be mapped to guest address space in a
> +way so that updates are visible to other processes mapping the
> +same memory region.
> +
> +\subsection{Driver Initialization}\label{sec:Device Types / PMEM Driver /
> Driver Initialization}
> +
> +Memory stores to the persistent memory range are not guaranteed to be
> +persistent without further action. An explicit flush command is
> +required to ensure persistence. The req_vq is used to perform flush
> +commands.
> +
> +\subsection{Driver Operations}\label{sec:Device Types / PMEM Driver / Driver
> Operation}
> +
> +The VIRTIO_PMEM_REQ_TYPE_FLUSH command persists all memory writes that were
> performed
> +before the command was submitted. Once the command completes those writes
> are guaranteed
> +to be persistent.
> +
> +\drivernormative{\subsubsection}{Driver Operation: Virtqueue command}{Device
> Types / PMEM Driver / Driver Operation / Virtqueue command}
> +
> +The driver MUST submit a VIRTIO_PMEM_REQ_TYPE_FLUSH command after performing
> memory writes
> +that require persistence.
> +
> +The driver MUST wait for the VIRTIO_PMEM_REQ_TYPE_FLUSH command to complete
> before
> +assuming previous writes are persistent.
> +
> +\subsection{Device Operations}\label{sec:Device Types / PMEM Driver / Device
> Operation}
> +
> +\devicenormative{\subsubsection}{Device Operations: Virtqueue flush}{Device
> Types / PMEM Device / Device Operation / Virtqueue flush}
> +
> +The device SHOULD handle multiple flush requests simultaneously using
> +corresponding host flush mechanisms.
> +
> +\devicenormative{\subsubsection}{Device operations: Virtqueue return}{Device
> Types / PMEM Device / Device Operation / Virtqueue return}
> +
> +The device MUST return integer '0' for success and '!0' for failure.
> +
> +\subsection{Possible security implications}\label{sec:Device Types / PMEM
> Device / Possible Security Implications}
> +
> +Two devices actually sharing the same memory creates a potential information
> +leak; as access patterns of one driver could be observable by another
> driver.
> +
> +This can happen for example if two devices are implemented in software
> +by a hypervisor, and two drivers are parts of VMs running on the
> +hypervisor. In this case, the timing of access to device memory
> +might leak information about access patterns from one VM to another.
> +
> +This can include, but might not be limited to:
> +\begin{enumerate}
> +\item Configurations sharing a single region of device memory (even in a
> read-only configuration)
> +\item Configurations with a shared cache between devices (e.g. Linux page
> cache)
> +\item Configurations with memory deduplication techniques such as KSM;
> similar side-channels
> + might be present if the device memory is shared with another system, e.g.
> information about
> + the hypervisor/host page cache might leak into a VM guest.
> +\end{enumerate}
> +
> +\subsection{Countermeasures}\label{sec:Device Types / PMEM Device / Possible
> Security Implications / Countermeasures}
> +Solution is to avoid sharing resources between devices.
> +\begin{enumerate}
> +\item Each VM must have its own device memory, not shared with any other VM
> or process.
> +\item If the VM workload is a special application and there is no risk, it
> is okay to share the device memory.
> +\item Don't allow host cache eviction from VM when device memory is shared
> with other VM or host process.
> +\end{enumerate}
> --
> 2.14.5
>
>
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