[virtio-comment] [PATCH v5 0/1] virtio-pmem: Support describing pmem as shared memory region

[virtio-comment] [PATCH v5 0/1] virtio-pmem: Support describing pmem as shared memory region

[tstark]

From: Taylor Stark <tstark@microsoft.com>

Changes from v4 [1]:
 - Clarified which fields of virtio_pmem_config are valid depending on shared
   memory feature negotiation (suggested by Cornelia Huck).

Changes from v3 [1]:
 - Rebased onto master now that the base virtio-pmem spec has been merged.

Changes from v2:
 - Incorporated suggestions from Cornelia Huck on rewording driver initialization.

Changes from v1:
 - Added in a feature bit (VIRTIO_PMEM_F_SHMEM_REGION) for controlling how the
   device indicates the guest physical address ranges to the driver. This feature
   directly affects control flow of the driver, since it seemed weird to have
   the driver indicate support for shared memory regions, and then needing
   to include an enum (or similar) informing the driver how the device
   indicated guest physical address ranges. If devices want to indicate the
   ranges as guest absolute addresses, they can skip negotiating the feature.
 - The linux driver implementation has been updated and tested, but I'm holding
   off on posting the patches to get some feedback on the new approach.
 - Moved some changes to proper subsections (normative subsections).

[1] https://lists.oasis-open.org/archives/virtio-comment/202111/msg00004.html

---

This patch updates the virtio-pmem spec to add support for describing the pmem
region as a shared memory region. This is required to support virtio-pmem in
Hyper-V, since Hyper-V only allows PCI devices to operate on memory ranges
defined via BARs. When using the virtio PCI transport, shared memory regions
are described via PCI BARs.

As virtio-pmem hasn't been added to the virtio spec yet (see this issue [1]),
this patch is based off the RFC spec [2]. The linux driver implementation has
been posted at [3].

[1] https://github.com/oasis-tcs/virtio-spec/issues/78
[2] https://lists.oasis-open.org/archives/virtio-dev/201903/msg00083.html
[3] https://lore.kernel.org/nvdimm/20210715223505.GA29329@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net

Taylor Stark (1):
  virtio-pmem: Support describing pmem as shared memory region

 conformance.tex | 14 ++++++++++++--
 virtio-pmem.tex | 40 +++++++++++++++++++++++++++++++++-------
 2 files changed, 45 insertions(+), 9 deletions(-)

-- 
2.33.0.vfs.0.0


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[virtio-comment] [PATCH v5 1/1] virtio-pmem: Support describing pmem as shared memory region

[tstark]

From: Taylor Stark <tstark@microsoft.com>

Update the virtio-pmem spec to add support for describing the pmem region as a
shared memory window. This is required to support virtio-pmem in Hyper-V, since
Hyper-V only allows PCI devices to operate on memory ranges defined via BARs.
When using the virtio PCI transport, shared memory regions are described via
PCI BARs.

Signed-off-by: Taylor Stark <tstark@microsoft.com>
---
 conformance.tex | 14 ++++++++++++--
 virtio-pmem.tex | 40 +++++++++++++++++++++++++++++++++-------
 2 files changed, 45 insertions(+), 9 deletions(-)

diff --git a/conformance.tex b/conformance.tex
index 80547db..d9f2b45 100644
--- a/conformance.tex
+++ b/conformance.tex
@@ -31,8 +31,9 @@ \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},
-\ref{sec:Conformance / Driver Conformance / SCMI Driver Conformance} or
-\ref{sec:Conformance / Driver Conformance / GPIO Driver Conformance}.
+\ref{sec:Conformance / Driver Conformance / SCMI Driver Conformance},
+\ref{sec:Conformance / Driver Conformance / GPIO 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}
@@ -314,6 +315,14 @@ \section{Conformance Targets}\label{sec:Conformance / Conformance Targets}
 \item \ref{drivernormative:Device Types / GPIO Device / eventq Operation}
 \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 Device / Device Initialization}
+\end{itemize}
+
 \conformance{\section}{Device Conformance}\label{sec:Conformance / Device Conformance}
 
 A device MUST conform to the following normative statements:
@@ -578,6 +587,7 @@ \section{Conformance Targets}\label{sec:Conformance / Conformance Targets}
 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}
diff --git a/virtio-pmem.tex b/virtio-pmem.tex
index 93ab3c1..384eaa3 100644
--- a/virtio-pmem.tex
+++ b/virtio-pmem.tex
@@ -24,7 +24,10 @@ \subsection{Virtqueues}\label{sec:Device Types / PMEM Device / Virtqueues}
 
 \subsection{Feature bits}\label{sec:Device Types / PMEM Device / Feature bits}
 
-There are currently no feature bits defined for this device.
+\begin{description}
+\item[VIRTIO_PMEM_F_SHMEM_REGION (0)] The guest physical address range will be
+indicated as a shared memory region.
+\end{description}
 
 \subsection{Device configuration layout}\label{sec:Device Types / PMEM Device / Device configuration layout}
 
@@ -36,22 +39,45 @@ \subsection{Device configuration layout}\label{sec:Device Types / PMEM Device /
 \end{lstlisting}
 
 \begin{description}
-\item[\field{start}] contains the physical address of the first byte of the persistent memory region.
+\item[\field{start}] contains the physical address of the first byte of the
+persistent memory region, if VIRTIO_PMEM_F_SHMEM_REGION has not been negotiated.
 
-\item[\field{size}] contains the length of this address range.
+\item[\field{size}] contains the length of this address range, if
+VIRTIO_PMEM_F_SHMEM_REGION has not been negotiated.
 \end{description}
 
+\subsection{Device Initialization}\label{sec:Device Types / PMEM Device / Device Initialization}
+
+The device indicates the guest physical address to the driver in one of two ways:
 \begin{enumerate}
-\item Driver vpmem start is read from \field{start}.
-\item Driver vpmem end is read from \field{size}.
+\item As a guest absolute address, using virtio_pmem_config.
+\item As a shared memory region.
 \end{enumerate}
 
-\subsection{Driver Initialization}\label{sec:Device Types / PMEM Driver / Driver Initialization}
-
 The driver determines the start address and size of the persistent memory region in preparation for reading or writing data.
 
 The driver initializes req_vq in preparation for making flush requests.
 
+\devicenormative{\subsubsection}{Device Initialization}{Device Types / PMEM Device / Device Initialization}
+
+If VIRTIO_PMEM_F_SHMEM_REGION has been negotiated, the device MUST indicate the
+guest physical address as a shared memory region. The device MUST use shared
+memory region ID 0. The device SHOULD set \field{start} and \field{size} to zero.
+
+If VIRTIO_PMEM_F_SHMEM_REGION has not been negotiated, the device MUST indicate
+the guest physical address as a guest absolute address. The device MUST set
+\field{start} to the absolute address and \field{size} to the size of the
+address range, in bytes.
+
+\drivernormative{\subsubsection}{Device Initialization}{Device Types / PMEM Device / Device Initialization}
+
+If VIRTIO_PMEM_F_SHMEM_REGION has been negotiated, the driver MUST query
+shared memory ID 0 for the physical address ranges, and MUST NOT use
+\field{start} or \field{stop}.
+
+If VIRTIO_PMEM_F_SHMEM_REGION has not been negotiated, the driver MUST read the
+physical address ranges from \field{start} and \field{stop}.
+
 \subsection{Driver Operations}\label{sec:Device Types / PMEM Driver / Driver Operation / Request Queues}
 
 Requests have the following format:
-- 
2.33.0.vfs.0.0


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Re: [virtio-comment] [PATCH v5 1/1] virtio-pmem: Support describing pmem as shared memory region

[Cornelia Huck]

On Wed, Nov 10 2021, tstark@linux.microsoft.com wrote:

> From: Taylor Stark <tstark@microsoft.com>
>
> Update the virtio-pmem spec to add support for describing the pmem region as a
> shared memory window. This is required to support virtio-pmem in Hyper-V, since
> Hyper-V only allows PCI devices to operate on memory ranges defined via BARs.
> When using the virtio PCI transport, shared memory regions are described via
> PCI BARs.
>
> Signed-off-by: Taylor Stark <tstark@microsoft.com>
> ---
>  conformance.tex | 14 ++++++++++++--
>  virtio-pmem.tex | 40 +++++++++++++++++++++++++++++++++-------
>  2 files changed, 45 insertions(+), 9 deletions(-)

This looks good to me now.

Could you please open an issue at
https://github.com/oasis-tcs/virtio-spec/issues to get this included? If
nobody has any further comments, I think we could start the voting
process soon.


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Re: [virtio-comment] [PATCH v5 1/1] virtio-pmem: Support describing pmem as shared memory region

[Taylor Stark]

On Thu, Nov 11, 2021 at 11:52:42AM +0100, Cornelia Huck wrote:
> On Wed, Nov 10 2021, tstark@linux.microsoft.com wrote:
> 
> > From: Taylor Stark <tstark@microsoft.com>
> >
> > Update the virtio-pmem spec to add support for describing the pmem region as a
> > shared memory window. This is required to support virtio-pmem in Hyper-V, since
> > Hyper-V only allows PCI devices to operate on memory ranges defined via BARs.
> > When using the virtio PCI transport, shared memory regions are described via
> > PCI BARs.
> >
> > Signed-off-by: Taylor Stark <tstark@microsoft.com>
> > ---
> >  conformance.tex | 14 ++++++++++++--
> >  virtio-pmem.tex | 40 +++++++++++++++++++++++++++++++++-------
> >  2 files changed, 45 insertions(+), 9 deletions(-)
> 
> This looks good to me now.
> 
> Could you please open an issue at
> https://github.com/oasis-tcs/virtio-spec/issues to get this included? If
> nobody has any further comments, I think we could start the voting
> process soon.

Sounds good, thanks Cornelia!

Fixes: https://github.com/oasis-tcs/virtio-spec/issues/121

[virtio-comment] Re: [PATCH v5 1/1] virtio-pmem: Support describing pmem as shared memory region

[Pankaj Gupta]

> Update the virtio-pmem spec to add support for describing the pmem region as a
> shared memory window. This is required to support virtio-pmem in Hyper-V, since
> Hyper-V only allows PCI devices to operate on memory ranges defined via BARs.
> When using the virtio PCI transport, shared memory regions are described via
> PCI BARs.
>
> Signed-off-by: Taylor Stark <tstark@microsoft.com>
> ---
>  conformance.tex | 14 ++++++++++++--
>  virtio-pmem.tex | 40 +++++++++++++++++++++++++++++++++-------
>  2 files changed, 45 insertions(+), 9 deletions(-)
>
> diff --git a/conformance.tex b/conformance.tex
> index 80547db..d9f2b45 100644
> --- a/conformance.tex
> +++ b/conformance.tex
> @@ -31,8 +31,9 @@ \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},
> -\ref{sec:Conformance / Driver Conformance / SCMI Driver Conformance} or
> -\ref{sec:Conformance / Driver Conformance / GPIO Driver Conformance}.
> +\ref{sec:Conformance / Driver Conformance / SCMI Driver Conformance},
> +\ref{sec:Conformance / Driver Conformance / GPIO 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}
> @@ -314,6 +315,14 @@ \section{Conformance Targets}\label{sec:Conformance / Conformance Targets}
>  \item \ref{drivernormative:Device Types / GPIO Device / eventq Operation}
>  \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 Device / Device Initialization}
> +\end{itemize}
> +
>  \conformance{\section}{Device Conformance}\label{sec:Conformance / Device Conformance}
>
>  A device MUST conform to the following normative statements:
> @@ -578,6 +587,7 @@ \section{Conformance Targets}\label{sec:Conformance / Conformance Targets}
>  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}
> diff --git a/virtio-pmem.tex b/virtio-pmem.tex
> index 93ab3c1..384eaa3 100644
> --- a/virtio-pmem.tex
> +++ b/virtio-pmem.tex
> @@ -24,7 +24,10 @@ \subsection{Virtqueues}\label{sec:Device Types / PMEM Device / Virtqueues}
>
>  \subsection{Feature bits}\label{sec:Device Types / PMEM Device / Feature bits}
>
> -There are currently no feature bits defined for this device.
> +\begin{description}
> +\item[VIRTIO_PMEM_F_SHMEM_REGION (0)] The guest physical address range will be
> +indicated as a shared memory region.
> +\end{description}
>
>  \subsection{Device configuration layout}\label{sec:Device Types / PMEM Device / Device configuration layout}
>
> @@ -36,22 +39,45 @@ \subsection{Device configuration layout}\label{sec:Device Types / PMEM Device /
>  \end{lstlisting}
>
>  \begin{description}
> -\item[\field{start}] contains the physical address of the first byte of the persistent memory region.
> +\item[\field{start}] contains the physical address of the first byte of the
> +persistent memory region, if VIRTIO_PMEM_F_SHMEM_REGION has not been negotiated.
>
> -\item[\field{size}] contains the length of this address range.
> +\item[\field{size}] contains the length of this address range, if
> +VIRTIO_PMEM_F_SHMEM_REGION has not been negotiated.
>  \end{description}
>
> +\subsection{Device Initialization}\label{sec:Device Types / PMEM Device / Device Initialization}
> +
> +The device indicates the guest physical address to the driver in one of two ways:
>  \begin{enumerate}
> -\item Driver vpmem start is read from \field{start}.
> -\item Driver vpmem end is read from \field{size}.
> +\item As a guest absolute address, using virtio_pmem_config.
> +\item As a shared memory region.
>  \end{enumerate}
>
> -\subsection{Driver Initialization}\label{sec:Device Types / PMEM Driver / Driver Initialization}
> -
>  The driver determines the start address and size of the persistent memory region in preparation for reading or writing data.
>
>  The driver initializes req_vq in preparation for making flush requests.
>
> +\devicenormative{\subsubsection}{Device Initialization}{Device Types / PMEM Device / Device Initialization}
> +
> +If VIRTIO_PMEM_F_SHMEM_REGION has been negotiated, the device MUST indicate the
> +guest physical address as a shared memory region. The device MUST use shared
> +memory region ID 0. The device SHOULD set \field{start} and \field{size} to zero.
> +
> +If VIRTIO_PMEM_F_SHMEM_REGION has not been negotiated, the device MUST indicate
> +the guest physical address as a guest absolute address. The device MUST set
> +\field{start} to the absolute address and \field{size} to the size of the
> +address range, in bytes.
> +
> +\drivernormative{\subsubsection}{Device Initialization}{Device Types / PMEM Device / Device Initialization}
> +
> +If VIRTIO_PMEM_F_SHMEM_REGION has been negotiated, the driver MUST query
> +shared memory ID 0 for the physical address ranges, and MUST NOT use
> +\field{start} or \field{stop}.
> +
> +If VIRTIO_PMEM_F_SHMEM_REGION has not been negotiated, the driver MUST read the
> +physical address ranges from \field{start} and \field{stop}.
> +
>  \subsection{Driver Operations}\label{sec:Device Types / PMEM Driver / Driver Operation / Request Queues}
>
>  Requests have the following format:

Looks good to me.

Reviewed-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>

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Re: [virtio-comment] [PATCH v5 1/1] virtio-pmem: Support describing pmem as shared memory region

[Halil Pasic]

On Wed, 10 Nov 2021 10:55:55 -0800
tstark@linux.microsoft.com wrote:
[..]
> @@ -36,22 +39,45 @@ \subsection{Device configuration layout}\label{sec:Device Types / PMEM Device /
>  \end{lstlisting}
>  
>  \begin{description}
> -\item[\field{start}] contains the physical address of the first byte of the persistent memory region.
> +\item[\field{start}] contains the physical address of the first byte of the
> +persistent memory region, if VIRTIO_PMEM_F_SHMEM_REGION has not been negotiated.
>  
> -\item[\field{size}] contains the length of this address range.
> +\item[\field{size}] contains the length of this address range, if
> +VIRTIO_PMEM_F_SHMEM_REGION has not been negotiated.
>  \end{description}
>  
> +\subsection{Device Initialization}\label{sec:Device Types / PMEM Device / Device Initialization}
> +
> +The device indicates the guest physical address to the driver in one of two ways:
>  \begin{enumerate}
> -\item Driver vpmem start is read from \field{start}.
> -\item Driver vpmem end is read from \field{size}.
> +\item As a guest absolute address, using virtio_pmem_config.
> +\item As a shared memory region.
>  \end{enumerate}
>  
> -\subsection{Driver Initialization}\label{sec:Device Types / PMEM Driver / Driver Initialization}
> -
>  The driver determines the start address and size of the persistent memory region in preparation for reading or writing data.
>  
>  The driver initializes req_vq in preparation for making flush requests.
>  
> +\devicenormative{\subsubsection}{Device Initialization}{Device Types / PMEM Device / Device Initialization}
> +
> +If VIRTIO_PMEM_F_SHMEM_REGION has been negotiated, the device MUST indicate the
> +guest physical address as a shared memory region. The device MUST use shared
> +memory region ID 0. The device SHOULD set \field{start} and \field{size} to zero.
> +
> +
> +If VIRTIO_PMEM_F_SHMEM_REGION has not been negotiated, the device MUST indicate
> +the guest physical address as a guest absolute address. The device MUST set
> +\field{start} to the absolute address and \field{size} to the size of the
> +address range, in bytes.

Sorry for joining in this late. 

I'm wondering if the quoted parts implies that the device SHOULD change
its config space (fields start and size) when the driver sets
FEATURES_OK, and the feature is VIRTIO_PMEM_F_SHMEM_REGION set at the
device (it was offered, and got set).

My train of thought is: initially no feature is considered negotiated,
and config space access is allowed before feature negotiation is
completed. That means the at this stage the device must indicate via
the config space, and thus \field{size} != 0. But as a part of the
transition 'features not negotiated' -> 'features negotiated' the device
needs to go '\field{size} != 0' -> '\field{size} == 0'

Is that what we want?

Also I understand that Hyper-V would make the device cease operation if
the feature VIRTIO_PMEM_F_SHMEM_REGION was not accepted by the driver.
I believe I've read that in some previous email.

Do we need a  'MAY fail to operate further if' statement? Anything that
ain't guarded by feature bits ain't optional, and anything that is
guarded by feature bits is optional unless explicitly stated otherwise.

Regards,
Halil

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Re: [virtio-comment] [PATCH v5 1/1] virtio-pmem: Support describing pmem as shared memory region

[Taylor Stark]

On Thu, Nov 18, 2021 at 06:37:52PM +0100, Halil Pasic wrote:
> On Wed, 10 Nov 2021 10:55:55 -0800
> tstark@linux.microsoft.com wrote:
> [..]
> > @@ -36,22 +39,45 @@ \subsection{Device configuration layout}\label{sec:Device Types / PMEM Device /
> >  \end{lstlisting}
> >  
> >  \begin{description}
> > -\item[\field{start}] contains the physical address of the first byte of the persistent memory region.
> > +\item[\field{start}] contains the physical address of the first byte of the
> > +persistent memory region, if VIRTIO_PMEM_F_SHMEM_REGION has not been negotiated.
> >  
> > -\item[\field{size}] contains the length of this address range.
> > +\item[\field{size}] contains the length of this address range, if
> > +VIRTIO_PMEM_F_SHMEM_REGION has not been negotiated.
> >  \end{description}
> >  
> > +\subsection{Device Initialization}\label{sec:Device Types / PMEM Device / Device Initialization}
> > +
> > +The device indicates the guest physical address to the driver in one of two ways:
> >  \begin{enumerate}
> > -\item Driver vpmem start is read from \field{start}.
> > -\item Driver vpmem end is read from \field{size}.
> > +\item As a guest absolute address, using virtio_pmem_config.
> > +\item As a shared memory region.
> >  \end{enumerate}
> >  
> > -\subsection{Driver Initialization}\label{sec:Device Types / PMEM Driver / Driver Initialization}
> > -
> >  The driver determines the start address and size of the persistent memory region in preparation for reading or writing data.
> >  
> >  The driver initializes req_vq in preparation for making flush requests.
> >  
> > +\devicenormative{\subsubsection}{Device Initialization}{Device Types / PMEM Device / Device Initialization}
> > +
> > +If VIRTIO_PMEM_F_SHMEM_REGION has been negotiated, the device MUST indicate the
> > +guest physical address as a shared memory region. The device MUST use shared
> > +memory region ID 0. The device SHOULD set \field{start} and \field{size} to zero.
> > +
> > +
> > +If VIRTIO_PMEM_F_SHMEM_REGION has not been negotiated, the device MUST indicate
> > +the guest physical address as a guest absolute address. The device MUST set
> > +\field{start} to the absolute address and \field{size} to the size of the
> > +address range, in bytes.
> 
> Sorry for joining in this late. 
> 
> I'm wondering if the quoted parts implies that the device SHOULD change
> its config space (fields start and size) when the driver sets
> FEATURES_OK, and the feature is VIRTIO_PMEM_F_SHMEM_REGION set at the
> device (it was offered, and got set).
> 
> My train of thought is: initially no feature is considered negotiated,
> and config space access is allowed before feature negotiation is
> completed. That means the at this stage the device must indicate via
> the config space, and thus \field{size} != 0. But as a part of the
> transition 'features not negotiated' -> 'features negotiated' the device
> needs to go '\field{size} != 0' -> '\field{size} == 0'
> 
> Is that what we want?
> 
> Also I understand that Hyper-V would make the device cease operation if
> the feature VIRTIO_PMEM_F_SHMEM_REGION was not accepted by the driver.
> I believe I've read that in some previous email.
> 
> Do we need a  'MAY fail to operate further if' statement? Anything that
> ain't guarded by feature bits ain't optional, and anything that is
> guarded by feature bits is optional unless explicitly stated otherwise.
> 
> Regards,
> Halil

No problem, thanks for joining in! :) I think your train of thought makes sense.
You're right, for Hyper-V we can't support any driver that doesn't support
VIRTIO_PMEM_F_SHMEM_REGION. So we have no choice but to set the config fields
to zero from the start. There's no meaningful value we can place in those fields.

I'm trying to figure out how best to word this. Saying that devices should set
config fields prior to FEATURE_OK is redundant. Should I add a section saying that
config fields MAY be set to 0 if VIRTIO_PMEM_F_SHMEM_REGION is offerred (such as
in the Hyper-V case)? And that if it's set to 0 and the feature isn't negotiated,
the device must reject the driver? I'd also have to add in a section saying that
the pmem region shouldn't be touched by the driver prior to DRIVER_OK being set,
to save drivers from accessing the zeroed values placed in the config. Additionally,
encouraging devices to set the config fields to 0 once the feature has been
negotiated seems like the right thing to do, to catch bugs?

I'm not sure about the 'MAY fail to operate further statement'. Couldn't there
be lots of reasons why a device rejects a driver? And wouldn't those be
implementation details of each device? Is it common to document those?

Thoughts?

Thanks!
Taylor

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Re: [virtio-comment] [PATCH v5 1/1] virtio-pmem: Support describing pmem as shared memory region

[Cornelia Huck]

On Sun, Nov 21 2021, Taylor Stark <tstark@linux.microsoft.com> wrote:

> On Thu, Nov 18, 2021 at 06:37:52PM +0100, Halil Pasic wrote:
>> On Wed, 10 Nov 2021 10:55:55 -0800
>> tstark@linux.microsoft.com wrote:
>> [..]
>> > @@ -36,22 +39,45 @@ \subsection{Device configuration layout}\label{sec:Device Types / PMEM Device /
>> >  \end{lstlisting}
>> >  
>> >  \begin{description}
>> > -\item[\field{start}] contains the physical address of the first byte of the persistent memory region.
>> > +\item[\field{start}] contains the physical address of the first byte of the
>> > +persistent memory region, if VIRTIO_PMEM_F_SHMEM_REGION has not been negotiated.
>> >  
>> > -\item[\field{size}] contains the length of this address range.
>> > +\item[\field{size}] contains the length of this address range, if
>> > +VIRTIO_PMEM_F_SHMEM_REGION has not been negotiated.
>> >  \end{description}
>> >  
>> > +\subsection{Device Initialization}\label{sec:Device Types / PMEM Device / Device Initialization}
>> > +
>> > +The device indicates the guest physical address to the driver in one of two ways:
>> >  \begin{enumerate}
>> > -\item Driver vpmem start is read from \field{start}.
>> > -\item Driver vpmem end is read from \field{size}.
>> > +\item As a guest absolute address, using virtio_pmem_config.
>> > +\item As a shared memory region.
>> >  \end{enumerate}
>> >  
>> > -\subsection{Driver Initialization}\label{sec:Device Types / PMEM Driver / Driver Initialization}
>> > -
>> >  The driver determines the start address and size of the persistent memory region in preparation for reading or writing data.
>> >  
>> >  The driver initializes req_vq in preparation for making flush requests.
>> >  
>> > +\devicenormative{\subsubsection}{Device Initialization}{Device Types / PMEM Device / Device Initialization}
>> > +
>> > +If VIRTIO_PMEM_F_SHMEM_REGION has been negotiated, the device MUST indicate the
>> > +guest physical address as a shared memory region. The device MUST use shared
>> > +memory region ID 0. The device SHOULD set \field{start} and \field{size} to zero.
>> > +
>> > +
>> > +If VIRTIO_PMEM_F_SHMEM_REGION has not been negotiated, the device MUST indicate

[for the sake of the argument made below, note the 'has not been
negotiated' here]

>> > +the guest physical address as a guest absolute address. The device MUST set
>> > +\field{start} to the absolute address and \field{size} to the size of the
>> > +address range, in bytes.
>> 
>> Sorry for joining in this late. 
>> 
>> I'm wondering if the quoted parts implies that the device SHOULD change
>> its config space (fields start and size) when the driver sets
>> FEATURES_OK, and the feature is VIRTIO_PMEM_F_SHMEM_REGION set at the
>> device (it was offered, and got set).
>> 
>> My train of thought is: initially no feature is considered negotiated,
>> and config space access is allowed before feature negotiation is
>> completed. That means the at this stage the device must indicate via
>> the config space, and thus \field{size} != 0. But as a part of the
>> transition 'features not negotiated' -> 'features negotiated' the device
>> needs to go '\field{size} != 0' -> '\field{size} == 0'

It has been advised that it SHOULD put 0 there, but I don't consider
that a must, just a good idea.

>> 
>> Is that what we want?
>> 
>> Also I understand that Hyper-V would make the device cease operation if
>> the feature VIRTIO_PMEM_F_SHMEM_REGION was not accepted by the driver.
>> I believe I've read that in some previous email.
>> 
>> Do we need a  'MAY fail to operate further if' statement? Anything that
>> ain't guarded by feature bits ain't optional, and anything that is
>> guarded by feature bits is optional unless explicitly stated otherwise.

The device can always fail the setting of FEATURES_OK, see 2.2.2:

"The device SHOULD accept any valid subset of features the driver
accepts, otherwise it MUST fail to set the FEATURES_OK device status bit
when the driver writes it."

I'd argue that "SHMEM_REGION not set" can be considered a non-valid
subset by the device. (Also, there's a SHOULD in there.)

>> 
>> Regards,
>> Halil
>
> No problem, thanks for joining in! :) I think your train of thought makes sense.
> You're right, for Hyper-V we can't support any driver that doesn't support
> VIRTIO_PMEM_F_SHMEM_REGION. So we have no choice but to set the config fields
> to zero from the start. There's no meaningful value we can place in those fields.

I think this is fine: we only require the fields to have meaningful
contents if SHMEM_REGION has not been negotiated -- as long as the
device has not accepted FEATURES_OK from the driver, nothing has been
negotiated respectively not been negotiated.

>
> I'm trying to figure out how best to word this. Saying that devices should set
> config fields prior to FEATURE_OK is redundant. Should I add a section saying that
> config fields MAY be set to 0 if VIRTIO_PMEM_F_SHMEM_REGION is offerred (such as
> in the Hyper-V case)? And that if it's set to 0 and the feature isn't negotiated,
> the device must reject the driver? I'd also have to add in a section saying that
> the pmem region shouldn't be touched by the driver prior to DRIVER_OK being set,
> to save drivers from accessing the zeroed values placed in the config. Additionally,
> encouraging devices to set the config fields to 0 once the feature has been
> negotiated seems like the right thing to do, to catch bugs?

I don't think we can prohibit the driver from reading the config space,
that is explicitly allowed in the general case.

We probably can add a non-normative extra statement, though, that a
device may present zeroes in the config space before feature negotiation
has finished if it doesn't support !SHMEM_REGION. That would also give
the driver a way to discover that it doesn't make sense to continue
without negotiating SHMEM_REGION. No extra normative statement, but
should make it a bit more clear to those trying to implement this.

>
> I'm not sure about the 'MAY fail to operate further statement'. Couldn't there
> be lots of reasons why a device rejects a driver? And wouldn't those be
> implementation details of each device? Is it common to document those?

We probably don't need such a statement (see above).

As voting for this is open until tomorrow, we should quickly decide what
to do. My preference would be to merge it as-is and do a clarification
on top.


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Re: [virtio-comment] [PATCH v5 1/1] virtio-pmem: Support describing pmem as shared memory region

[Halil Pasic]

On Mon, 22 Nov 2021 12:58:01 +0100
Cornelia Huck <cohuck@redhat.com> wrote:

> On Sun, Nov 21 2021, Taylor Stark <tstark@linux.microsoft.com> wrote:
> 
> > On Thu, Nov 18, 2021 at 06:37:52PM +0100, Halil Pasic wrote:  
> >> On Wed, 10 Nov 2021 10:55:55 -0800
> >> tstark@linux.microsoft.com wrote:
> >> [..]  
> >> > @@ -36,22 +39,45 @@ \subsection{Device configuration layout}\label{sec:Device Types / PMEM Device /
> >> >  \end{lstlisting}
> >> >  
> >> >  \begin{description}
> >> > -\item[\field{start}] contains the physical address of the first byte of the persistent memory region.
> >> > +\item[\field{start}] contains the physical address of the first byte of the
> >> > +persistent memory region, if VIRTIO_PMEM_F_SHMEM_REGION has not been negotiated.
> >> >  
> >> > -\item[\field{size}] contains the length of this address range.
> >> > +\item[\field{size}] contains the length of this address range, if
> >> > +VIRTIO_PMEM_F_SHMEM_REGION has not been negotiated.
> >> >  \end{description}
> >> >  
> >> > +\subsection{Device Initialization}\label{sec:Device Types / PMEM Device / Device Initialization}
> >> > +
> >> > +The device indicates the guest physical address to the driver in one of two ways:
> >> >  \begin{enumerate}
> >> > -\item Driver vpmem start is read from \field{start}.
> >> > -\item Driver vpmem end is read from \field{size}.
> >> > +\item As a guest absolute address, using virtio_pmem_config.
> >> > +\item As a shared memory region.
> >> >  \end{enumerate}
> >> >  
> >> > -\subsection{Driver Initialization}\label{sec:Device Types / PMEM Driver / Driver Initialization}
> >> > -
> >> >  The driver determines the start address and size of the persistent memory region in preparation for reading or writing data.
> >> >  
> >> >  The driver initializes req_vq in preparation for making flush requests.
> >> >  
> >> > +\devicenormative{\subsubsection}{Device Initialization}{Device Types / PMEM Device / Device Initialization}
> >> > +
> >> > +If VIRTIO_PMEM_F_SHMEM_REGION has been negotiated, the device MUST indicate the
> >> > +guest physical address as a shared memory region. The device MUST use shared
> >> > +memory region ID 0. The device SHOULD set \field{start} and \field{size} to zero.
> >> > +
> >> > +
> >> > +If VIRTIO_PMEM_F_SHMEM_REGION has not been negotiated, the device MUST indicate  
> 
> [for the sake of the argument made below, note the 'has not been
> negotiated' here]
> 
> >> > +the guest physical address as a guest absolute address. The device MUST set
> >> > +\field{start} to the absolute address and \field{size} to the size of the
> >> > +address range, in bytes.  
> >> 
> >> Sorry for joining in this late. 
> >> 
> >> I'm wondering if the quoted parts implies that the device SHOULD change
> >> its config space (fields start and size) when the driver sets
> >> FEATURES_OK, and the feature is VIRTIO_PMEM_F_SHMEM_REGION set at the
> >> device (it was offered, and got set).
> >> 
> >> My train of thought is: initially no feature is considered negotiated,
> >> and config space access is allowed before feature negotiation is
> >> completed. That means the at this stage the device must indicate via
> >> the config space, and thus \field{size} != 0. But as a part of the
> >> transition 'features not negotiated' -> 'features negotiated' the device
> >> needs to go '\field{size} != 0' -> '\field{size} == 0'  
> 
> It has been advised that it SHOULD put 0 there, but I don't consider
> that a must, just a good idea.

Right I agree with that. So we say that the config state update is a
good idea as well?

BTW the definition of SHOULD:
   SHOULD   This word, or the adjective "RECOMMENDED", mean that there
   may exist valid reasons in particular circumstances to ignore a
   particular item, but the full implications must be understood and
   carefully weighed before choosing a different course.

Maybe I should have used 'should go' instead of 'needs to go' but that
doesn't change the fact, that the spec currently regards this
config space update as RECOMMENDED.

Conny, is that what we want?


> 
> >> 
> >> Is that what we want?
> >> 
> >> Also I understand that Hyper-V would make the device cease operation if
> >> the feature VIRTIO_PMEM_F_SHMEM_REGION was not accepted by the driver.
> >> I believe I've read that in some previous email.
> >> 
> >> Do we need a  'MAY fail to operate further if' statement? Anything that
> >> ain't guarded by feature bits ain't optional, and anything that is
> >> guarded by feature bits is optional unless explicitly stated otherwise.  
> 
> The device can always fail the setting of FEATURES_OK, see 2.2.2:
> 
> "The device SHOULD accept any valid subset of features the driver
> accepts, otherwise it MUST fail to set the FEATURES_OK device status bit
> when the driver writes it."
> 
> I'd argue that "SHMEM_REGION not set" can be considered a non-valid
> subset by the device. (Also, there's a SHOULD in there.)

Yes, but that is not very helpful for somebody reading the spec without
having the context of this discussion.

To put it differently, why do we use the "MAY fail to operate further
if" statement for:
* VIRTIO_F_ACCESS_PLATFORM not accepted
* VIRTIO_F_ORDER_PLATFORM not accepted
* VIRTIO_F_VERSION_1 not offered

If "SHMEM_REGION not accepted" is covered by 2.2.2 and thus superfluous,
then so is "ACCESS_PLATFORM not accepted".

IMHO in the sentence quoted by you "valid subset" should be interpreted
as a subset that is closed to the "depends on" relationship between
feature sets. I.e. a subset is invalid, if it contains at least one
feature such that some feature required by it is not in the subset.

If we say that any device implementation can proclaim any subset of
features invalid, than the the first part of the sentence does
not have any meaning to it.

You are definitively right about the SHOULD, it is not a MUST.

I believe the whole idea behind features is to facilitate extensibility
and compatibility.

Usually a driver can safely ignore features it does not know or care
about,and the device SHOULD still work according to the expectations of
the driver.

But some feature bits are not like that. For example it ain't safe for
the driver to ignore ACCESS_PLATFORM, for obvious reasons. And we
express that with the "MAY fail to operate further if" statement.


> 
> >> 
> >> Regards,
> >> Halil  
> >
> > No problem, thanks for joining in! :) I think your train of thought makes sense.
> > You're right, for Hyper-V we can't support any driver that doesn't support
> > VIRTIO_PMEM_F_SHMEM_REGION. So we have no choice but to set the config fields
> > to zero from the start. There's no meaningful value we can place in those fields.  
> 
> I think this is fine: we only require the fields to have meaningful
> contents if SHMEM_REGION has not been negotiated -- as long as the
> device has not accepted FEATURES_OK from the driver, nothing has been
> negotiated respectively not been negotiated.

I have issues accepting the tristate nature of 'has been negotiated'. I
don't think that is intuitive. If this really what we want we should be
very clear about it.

In my opinion feature negotiation is usually an opt-in. Before the opt-in
the device is to give us the "compatible" or "old" behavior.

> 
> >
> > I'm trying to figure out how best to word this. Saying that devices should set
> > config fields prior to FEATURE_OK is redundant. Should I add a section saying that
> > config fields MAY be set to 0 if VIRTIO_PMEM_F_SHMEM_REGION is offerred (such as
> > in the Hyper-V case)? And that if it's set to 0 and the feature isn't negotiated,
> > the device must reject the driver? I'd also have to add in a section saying that
> > the pmem region shouldn't be touched by the driver prior to DRIVER_OK being set,
> > to save drivers from accessing the zeroed values placed in the config. Additionally,
> > encouraging devices to set the config fields to 0 once the feature has been
> > negotiated seems like the right thing to do, to catch bugs?  
> 
> I don't think we can prohibit the driver from reading the config space,
> that is explicitly allowed in the general case.

I agree. I believe the specification should tell us which features are
essential by design, and which features are optional by design. We
clearly have both kinds of features, right?

> 
> We probably can add a non-normative extra statement, though, that a
> device may present zeroes in the config space before feature negotiation
> has finished if it doesn't support !SHMEM_REGION. 

I would not tie that to 'before feature negotiation' or 'does not support
SHMEM'. We can say something like:

If the value of the field \filed{size} is 0, then the \field{start} does
not contain the physical address of the persistent memory region, and
the persistent memory region is advertised by means different than 
via the config space.

> That would also give
> the driver a way to discover that it doesn't make sense to continue
> without negotiating SHMEM_REGION. No extra normative statement, but
> should make it a bit more clear to those trying to implement this.
> 
> >
> > I'm not sure about the 'MAY fail to operate further statement'. Couldn't there
> > be lots of reasons why a device rejects a driver? And wouldn't those be
> > implementation details of each device? Is it common to document those?  
> 
> We probably don't need such a statement (see above).

I disagree.

> 
> As voting for this is open until tomorrow, we should quickly decide what
> to do. My preference would be to merge it as-is and do a clarification
> on top.
> 

I'm fine with merging as is.

> 
> This publicly archived list offers a means to provide input to the
> OASIS Virtual I/O Device (VIRTIO) TC.
> 
> In order to verify user consent to the Feedback License terms and
> to minimize spam in the list archive, subscription is required
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> 
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> 

Re: [virtio-comment] [PATCH v5 1/1] virtio-pmem: Support describing pmem as shared memory region

[Cornelia Huck]

On Mon, Nov 22 2021, Halil Pasic <pasic@linux.ibm.com> wrote:

> On Mon, 22 Nov 2021 12:58:01 +0100
> Cornelia Huck <cohuck@redhat.com> wrote:
>
>> On Sun, Nov 21 2021, Taylor Stark <tstark@linux.microsoft.com> wrote:
>> 
>> > On Thu, Nov 18, 2021 at 06:37:52PM +0100, Halil Pasic wrote:  
>> >> On Wed, 10 Nov 2021 10:55:55 -0800
>> >> tstark@linux.microsoft.com wrote:
>> >> [..]  
>> >> > @@ -36,22 +39,45 @@ \subsection{Device configuration layout}\label{sec:Device Types / PMEM Device /
>> >> >  \end{lstlisting}
>> >> >  
>> >> >  \begin{description}
>> >> > -\item[\field{start}] contains the physical address of the first byte of the persistent memory region.
>> >> > +\item[\field{start}] contains the physical address of the first byte of the
>> >> > +persistent memory region, if VIRTIO_PMEM_F_SHMEM_REGION has not been negotiated.
>> >> >  
>> >> > -\item[\field{size}] contains the length of this address range.
>> >> > +\item[\field{size}] contains the length of this address range, if
>> >> > +VIRTIO_PMEM_F_SHMEM_REGION has not been negotiated.
>> >> >  \end{description}
>> >> >  
>> >> > +\subsection{Device Initialization}\label{sec:Device Types / PMEM Device / Device Initialization}
>> >> > +
>> >> > +The device indicates the guest physical address to the driver in one of two ways:
>> >> >  \begin{enumerate}
>> >> > -\item Driver vpmem start is read from \field{start}.
>> >> > -\item Driver vpmem end is read from \field{size}.
>> >> > +\item As a guest absolute address, using virtio_pmem_config.
>> >> > +\item As a shared memory region.
>> >> >  \end{enumerate}
>> >> >  
>> >> > -\subsection{Driver Initialization}\label{sec:Device Types / PMEM Driver / Driver Initialization}
>> >> > -
>> >> >  The driver determines the start address and size of the persistent memory region in preparation for reading or writing data.
>> >> >  
>> >> >  The driver initializes req_vq in preparation for making flush requests.
>> >> >  
>> >> > +\devicenormative{\subsubsection}{Device Initialization}{Device Types / PMEM Device / Device Initialization}
>> >> > +
>> >> > +If VIRTIO_PMEM_F_SHMEM_REGION has been negotiated, the device MUST indicate the
>> >> > +guest physical address as a shared memory region. The device MUST use shared
>> >> > +memory region ID 0. The device SHOULD set \field{start} and \field{size} to zero.
>> >> > +
>> >> > +
>> >> > +If VIRTIO_PMEM_F_SHMEM_REGION has not been negotiated, the device MUST indicate  
>> 
>> [for the sake of the argument made below, note the 'has not been
>> negotiated' here]
>> 
>> >> > +the guest physical address as a guest absolute address. The device MUST set
>> >> > +\field{start} to the absolute address and \field{size} to the size of the
>> >> > +address range, in bytes.  
>> >> 
>> >> Sorry for joining in this late. 
>> >> 
>> >> I'm wondering if the quoted parts implies that the device SHOULD change
>> >> its config space (fields start and size) when the driver sets
>> >> FEATURES_OK, and the feature is VIRTIO_PMEM_F_SHMEM_REGION set at the
>> >> device (it was offered, and got set).
>> >> 
>> >> My train of thought is: initially no feature is considered negotiated,
>> >> and config space access is allowed before feature negotiation is
>> >> completed. That means the at this stage the device must indicate via
>> >> the config space, and thus \field{size} != 0. But as a part of the
>> >> transition 'features not negotiated' -> 'features negotiated' the device
>> >> needs to go '\field{size} != 0' -> '\field{size} == 0'  
>> 
>> It has been advised that it SHOULD put 0 there, but I don't consider
>> that a must, just a good idea.
>
> Right I agree with that. So we say that the config state update is a
> good idea as well?
>
> BTW the definition of SHOULD:
>    SHOULD   This word, or the adjective "RECOMMENDED", mean that there
>    may exist valid reasons in particular circumstances to ignore a
>    particular item, but the full implications must be understood and
>    carefully weighed before choosing a different course.
>
> Maybe I should have used 'should go' instead of 'needs to go' but that
> doesn't change the fact, that the spec currently regards this
> config space update as RECOMMENDED.
>
> Conny, is that what we want?
>

I really don't understand your problem with this? Yes, we recommend that
the device does this, but if there's anything that makes it difficult for
the device, it can omit it.

>
>> 
>> >> 
>> >> Is that what we want?
>> >> 
>> >> Also I understand that Hyper-V would make the device cease operation if
>> >> the feature VIRTIO_PMEM_F_SHMEM_REGION was not accepted by the driver.
>> >> I believe I've read that in some previous email.
>> >> 
>> >> Do we need a  'MAY fail to operate further if' statement? Anything that
>> >> ain't guarded by feature bits ain't optional, and anything that is
>> >> guarded by feature bits is optional unless explicitly stated otherwise.  
>> 
>> The device can always fail the setting of FEATURES_OK, see 2.2.2:
>> 
>> "The device SHOULD accept any valid subset of features the driver
>> accepts, otherwise it MUST fail to set the FEATURES_OK device status bit
>> when the driver writes it."
>> 
>> I'd argue that "SHMEM_REGION not set" can be considered a non-valid
>> subset by the device. (Also, there's a SHOULD in there.)
>
> Yes, but that is not very helpful for somebody reading the spec without
> having the context of this discussion.
>
> To put it differently, why do we use the "MAY fail to operate further
> if" statement for:
> * VIRTIO_F_ACCESS_PLATFORM not accepted
> * VIRTIO_F_ORDER_PLATFORM not accepted
> * VIRTIO_F_VERSION_1 not offered
>
> If "SHMEM_REGION not accepted" is covered by 2.2.2 and thus superfluous,
> then so is "ACCESS_PLATFORM not accepted".

I'd say it is indeed not really needed. The driver must be able to deal
with the device rejecting FEATURES_OK for a certain feature set.

>
> IMHO in the sentence quoted by you "valid subset" should be interpreted
> as a subset that is closed to the "depends on" relationship between
> feature sets. I.e. a subset is invalid, if it contains at least one
> feature such that some feature required by it is not in the subset.
>
> If we say that any device implementation can proclaim any subset of
> features invalid, than the the first part of the sentence does
> not have any meaning to it.
>
> You are definitively right about the SHOULD, it is not a MUST.
>
> I believe the whole idea behind features is to facilitate extensibility
> and compatibility.
>
> Usually a driver can safely ignore features it does not know or care
> about,and the device SHOULD still work according to the expectations of
> the driver.
>
> But some feature bits are not like that. For example it ain't safe for
> the driver to ignore ACCESS_PLATFORM, for obvious reasons. And we
> express that with the "MAY fail to operate further if" statement.

Well if you want to highlight a bit, highlight it. But I don't think
this is really needed here. Things like ACCESS_PLATFORM are not as
obvious.

>
>
>> 
>> >> 
>> >> Regards,
>> >> Halil  
>> >
>> > No problem, thanks for joining in! :) I think your train of thought makes sense.
>> > You're right, for Hyper-V we can't support any driver that doesn't support
>> > VIRTIO_PMEM_F_SHMEM_REGION. So we have no choice but to set the config fields
>> > to zero from the start. There's no meaningful value we can place in those fields.  
>> 
>> I think this is fine: we only require the fields to have meaningful
>> contents if SHMEM_REGION has not been negotiated -- as long as the
>> device has not accepted FEATURES_OK from the driver, nothing has been
>> negotiated respectively not been negotiated.
>
> I have issues accepting the tristate nature of 'has been negotiated'. I
> don't think that is intuitive. If this really what we want we should be
> very clear about it.
>
> In my opinion feature negotiation is usually an opt-in. Before the opt-in
> the device is to give us the "compatible" or "old" behavior.

But a device is always free to require the "new" behaviour; otherwise,
we could never ditch old interfaces.

>
>> 
>> >
>> > I'm trying to figure out how best to word this. Saying that devices should set
>> > config fields prior to FEATURE_OK is redundant. Should I add a section saying that
>> > config fields MAY be set to 0 if VIRTIO_PMEM_F_SHMEM_REGION is offerred (such as
>> > in the Hyper-V case)? And that if it's set to 0 and the feature isn't negotiated,
>> > the device must reject the driver? I'd also have to add in a section saying that
>> > the pmem region shouldn't be touched by the driver prior to DRIVER_OK being set,
>> > to save drivers from accessing the zeroed values placed in the config. Additionally,
>> > encouraging devices to set the config fields to 0 once the feature has been
>> > negotiated seems like the right thing to do, to catch bugs?  
>> 
>> I don't think we can prohibit the driver from reading the config space,
>> that is explicitly allowed in the general case.
>
> I agree. I believe the specification should tell us which features are
> essential by design, and which features are optional by design. We
> clearly have both kinds of features, right?

It also depends on the individual device which features are essential.

>
>> 
>> We probably can add a non-normative extra statement, though, that a
>> device may present zeroes in the config space before feature negotiation
>> has finished if it doesn't support !SHMEM_REGION. 
>
> I would not tie that to 'before feature negotiation' or 'does not support
> SHMEM'. We can say something like:
>
> If the value of the field \filed{size} is 0, then the \field{start} does
> not contain the physical address of the persistent memory region, and
> the persistent memory region is advertised by means different than 
> via the config space.

Isn't that self-evident? If the device does not provide the information
in the config space, it uses a different mechanism.

>
>> That would also give
>> the driver a way to discover that it doesn't make sense to continue
>> without negotiating SHMEM_REGION. No extra normative statement, but
>> should make it a bit more clear to those trying to implement this.
>> 
>> >
>> > I'm not sure about the 'MAY fail to operate further statement'. Couldn't there
>> > be lots of reasons why a device rejects a driver? And wouldn't those be
>> > implementation details of each device? Is it common to document those?  
>> 
>> We probably don't need such a statement (see above).
>
> I disagree.

I still stand by my opinion.

>
>> 
>> As voting for this is open until tomorrow, we should quickly decide what
>> to do. My preference would be to merge it as-is and do a clarification
>> on top.
>> 
>
> I'm fine with merging as is.

OK; if everyone agrees we need changes on top, we can add them later.


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Re: [virtio-comment] [PATCH v5 1/1] virtio-pmem: Support describing pmem as shared memory region

[Stefan Hajnoczi]

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On Thu, Nov 11, 2021 at 11:52:42AM +0100, Cornelia Huck wrote:
> On Wed, Nov 10 2021, tstark@linux.microsoft.com wrote:
> 
> > From: Taylor Stark <tstark@microsoft.com>
> >
> > Update the virtio-pmem spec to add support for describing the pmem region as a
> > shared memory window. This is required to support virtio-pmem in Hyper-V, since
> > Hyper-V only allows PCI devices to operate on memory ranges defined via BARs.
> > When using the virtio PCI transport, shared memory regions are described via
> > PCI BARs.
> >
> > Signed-off-by: Taylor Stark <tstark@microsoft.com>
> > ---
> >  conformance.tex | 14 ++++++++++++--
> >  virtio-pmem.tex | 40 +++++++++++++++++++++++++++++++++-------
> >  2 files changed, 45 insertions(+), 9 deletions(-)
> 
> This looks good to me now.

I noticed the patch uses "guest absolute address" instead of "physical
address". This terminology was fixed in the original pmem patch and I
guess the shared memory patch wasn't updated. Please don't use "guest".

Stefan

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Re: [virtio-comment] [PATCH v5 1/1] virtio-pmem: Support describing pmem as shared memory region

[Cornelia Huck]

On Tue, Nov 23 2021, Stefan Hajnoczi <stefanha@redhat.com> wrote:

> On Thu, Nov 11, 2021 at 11:52:42AM +0100, Cornelia Huck wrote:
>> On Wed, Nov 10 2021, tstark@linux.microsoft.com wrote:
>> 
>> > From: Taylor Stark <tstark@microsoft.com>
>> >
>> > Update the virtio-pmem spec to add support for describing the pmem region as a
>> > shared memory window. This is required to support virtio-pmem in Hyper-V, since
>> > Hyper-V only allows PCI devices to operate on memory ranges defined via BARs.
>> > When using the virtio PCI transport, shared memory regions are described via
>> > PCI BARs.
>> >
>> > Signed-off-by: Taylor Stark <tstark@microsoft.com>
>> > ---
>> >  conformance.tex | 14 ++++++++++++--
>> >  virtio-pmem.tex | 40 +++++++++++++++++++++++++++++++++-------
>> >  2 files changed, 45 insertions(+), 9 deletions(-)
>> 
>> This looks good to me now.
>
> I noticed the patch uses "guest absolute address" instead of "physical
> address". This terminology was fixed in the original pmem patch and I
> guess the shared memory patch wasn't updated. Please don't use "guest".

So s/guest absolute/physical/ ? That can be fixed as an editorial update
on top.


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Re: [virtio-comment] [PATCH v5 1/1] virtio-pmem: Support describing pmem as shared memory region

[Stefan Hajnoczi]

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On Tue, Nov 23, 2021 at 05:49:58PM +0100, Cornelia Huck wrote:
> On Tue, Nov 23 2021, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> 
> > On Thu, Nov 11, 2021 at 11:52:42AM +0100, Cornelia Huck wrote:
> >> On Wed, Nov 10 2021, tstark@linux.microsoft.com wrote:
> >> 
> >> > From: Taylor Stark <tstark@microsoft.com>
> >> >
> >> > Update the virtio-pmem spec to add support for describing the pmem region as a
> >> > shared memory window. This is required to support virtio-pmem in Hyper-V, since
> >> > Hyper-V only allows PCI devices to operate on memory ranges defined via BARs.
> >> > When using the virtio PCI transport, shared memory regions are described via
> >> > PCI BARs.
> >> >
> >> > Signed-off-by: Taylor Stark <tstark@microsoft.com>
> >> > ---
> >> >  conformance.tex | 14 ++++++++++++--
> >> >  virtio-pmem.tex | 40 +++++++++++++++++++++++++++++++++-------
> >> >  2 files changed, 45 insertions(+), 9 deletions(-)
> >> 
> >> This looks good to me now.
> >
> > I noticed the patch uses "guest absolute address" instead of "physical
> > address". This terminology was fixed in the original pmem patch and I
> > guess the shared memory patch wasn't updated. Please don't use "guest".
> 
> So s/guest absolute/physical/ ? That can be fixed as an editorial update
> on top.
> 

Yes, please.

Stefan

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