[virtio-dev] RE: [PATCH v6 1/1] content: Add new feature VIRTIO_F_PRESERVE_RESOURCES

Hi Jiqian,

> From: Jiqian Chen <[email protected]>
> Sent: Saturday, October 21, 2023 9:21 AM
> To: Michael S . Tsirkin <[email protected]>; Gerd Hoffmann
> <[email protected]>; Parav Pandit <[email protected]>; Jason Wang
> <[email protected]>; Xuan Zhuo <[email protected]>; David
> Airlie <[email protected]>; Gurchetan Singh
> <[email protected]>; Chia-I Wu <[email protected]>; Marc-
> André Lureau <[email protected]>; Robert Beckett
> <[email protected]>; Mikhail Golubev-Ciuchea <Mikhail.Golubev-
> [email protected]>; [email protected]; virtio-
> [email protected]
> Cc: Honglei Huang <[email protected]>; Julia Zhang
> <[email protected]>; Huang Rui <[email protected]>; Jiqian Chen
> <[email protected]>
> Subject: [PATCH v6 1/1] content: Add new feature
> VIRTIO_F_PRESERVE_RESOURCES
> 
> In some scenes, Qemu may reset or destroy resources of virtio device, but some
> of them can't be re-created, so that causes some problems.
> 
It can be re-created. It is just that the guest driver lost the previous resource values.
So a better wording for the motivation is combining with below line to me is,

Currently guest drivers destroy and re-create virtio resources during power management suspend/resume sequence respectively.
For example, for a PCI transport, even if the device offers D3 to d0 state transition, a virtio guest driver has no way to know if 
the device can store the state during D0->D3 transition and same state can be restored on D3->D0 transition.

> For example, when we do S3 for guest, guest will set device_status to 0, it
> causes Qemu to reset virtioi-gpu device, and then all render resources of virtio-
> gpu will be destroyed. As a result, after guest resuming, the display can't come
> back, and we only see a black screen.
> 
To make guest drivers aware of above device capability, introduce a new feature bit to indicate such device capability.
This patch adds...

> In order to deal with the above scene, we need a mechanism that allows guest
> and Qemu to negotiate their behaviors for resources. So, this patch adds a new
> feature named VIRTIO_F_PRESERVE_RESOURCES. It allows guest to tell Qemu
> when there is a need to preserve resources, guest must preserve resources until
> 0 is set.
> 
I think this can be done without introducing the new register.
Can you please check if the PM register itself can serve the purpose instead of new virtio level register?

> Signed-off-by: Jiqian Chen <[email protected]>
> ---
>  conformance.tex   |  2 ++
>  content.tex       | 25 +++++++++++++++++++++++++
>  transport-pci.tex |  6 ++++++
>  3 files changed, 33 insertions(+)
> 
> diff --git a/conformance.tex b/conformance.tex index dc00e84..60cc0b1
> 100644
> --- a/conformance.tex
> +++ b/conformance.tex
> @@ -91,6 +91,7 @@ \section{Conformance Targets}\label{sec:Conformance /
> Conformance Targets}  \item \ref{drivernormative:Basic Facilities of a Virtio
> Device / Packed Virtqueues / The Virtqueue Descriptor Table / Indirect
> Descriptors}  \item \ref{drivernormative:Basic Facilities of a Virtio Device /
> Packed Virtqueues / Supplying Buffers to The Device / Updating flags}  \item
> \ref{drivernormative:Basic Facilities of a Virtio Device / Packed Virtqueues /
> Supplying Buffers to The Device / Sending Available Buffer Notifications}
> +\item \ref{drivernormative:Basic Facilities of a Virtio Device /
> +Preserve Resources}
>  \item \ref{drivernormative:General Initialization And Device Operation /
> Device Initialization}  \item \ref{drivernormative:General Initialization And
> Device Operation / Device Cleanup}  \item \ref{drivernormative:Reserved
> Feature Bits} @@ -172,6 +173,7 @@ \section{Conformance
> Targets}\label{sec:Conformance / Conformance Targets}  \item
> \ref{devicenormative:Basic Facilities of a Virtio Device / Packed Virtqueues /
> The Virtqueue Descriptor Table}  \item \ref{devicenormative:Basic Facilities of
> a Virtio Device / Packed Virtqueues / Scatter-Gather Support}  \item
> \ref{devicenormative:Basic Facilities of a Virtio Device / Shared Memory
> Regions}
> +\item \ref{devicenormative:Basic Facilities of a Virtio Device /
> +Preserve Resources}
>  \item \ref{devicenormative:Reserved Feature Bits}  \end{itemize}
> 
> diff --git a/content.tex b/content.tex
> index 0a62dce..b6b1859 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -502,6 +502,27 @@ \section{Exporting Objects}\label{sec:Basic Facilities
> of a Virtio Device / Expo  types. It is RECOMMENDED that devices generate
> version 4  UUIDs as specified by \hyperref[intro:rfc4122]{[RFC4122]}.
> 
> +\section{Preserve Resources}\label{sec:Basic Facilities of a Virtio
> +Device / Preserve Resources}
> +
> +As virtio devices are paravirtualization devices by design.
This is not true and not relevant for the spec. Please remove this line.

> +There are various devices resources created by sending commands from
> +frontend and stored in backend.
> +
> +In some scenes, resources may be destroyed or reset, some of them can
> +be re-created since frontend has enough information , but some can't.
> +At this case, we can set \field{Preserve Resources} to 1 by specific
> +transport, to prevent resources being destroyed.
> +
> +Which kind of resources need to be preserved and how to preserve
> +resources depend on specific devices.
s/on specific devices/on specific device type/

> +
> +\drivernormative{\subsection}{Preserve Resources}{Basic Facilities of a
> +Virtio Device / Preserve resources} A driver SHOULD set \field{Preserve
> +Resources} to 1 when there is a need to preserve resources.
> +
> +\devicenormative{\subsection}{Preserve Resources}{Basic Facilities of a
> +Virtio Device / Preserve resources} A device MUST NOT destroy resources until
> \field{Preserve Resources} is 0.
> +
>  \input{admin.tex}
> 
>  \chapter{General Initialization And Device Operation}\label{sec:General
> Initialization And Device Operation} @@ -872,6 +893,10 @@
> \chapter{Reserved Feature Bits}\label{sec:Reserved Feature Bits}
>  	\ref{devicenormative:Basic Facilities of a Virtio Device / Feature Bits}
> for
>  	handling features reserved for future use.
> 
> +  \item[VIRTIO_F_PRESERVE_RESOURCES(42)] This feature indicates  that
> + the device need to preserve resources.
> +  See \ref{sec:Basic Facilities of a Virtio Device / Preserve Resources}.
> +
>  \end{description}
> 
>  \drivernormative{\section}{Reserved Feature Bits}{Reserved Feature Bits} diff --
> git a/transport-pci.tex b/transport-pci.tex index a5c6719..f6eea65 100644
> --- a/transport-pci.tex
> +++ b/transport-pci.tex
> @@ -325,6 +325,7 @@ \subsubsection{Common configuration structure
> layout}\label{sec:Virtio Transport
>          /* About the administration virtqueue. */
>          le16 admin_queue_index;         /* read-only for driver */
>          le16 admin_queue_num;         /* read-only for driver */
> +        le16 preserve_resources;        /* read-write */
Preserving these resources in the device implementation takes finite amount of time.
Possibly more than 40nsec (time of PCIe write TLP).
Hence this register must be a polling register to indicate that preservation_done.
This will tell the guest when the preservation is done, and when restoration is done, so that it can resume upper layers.

Please refer to queue_reset definition to learn more about such register definition.

Lets please make sure that PCIe PM level registers are sufficient/not-sufficient to decide the addition of this register.

>  };
>  \end{lstlisting}
> 
> @@ -428,6 +429,11 @@ \subsubsection{Common configuration structure
> layout}\label{sec:Virtio Transport
>  	The value 0 indicates no supported administration virtqueues.
>  	This field is valid only if VIRTIO_F_ADMIN_VQ has been
>  	negotiated.
> +
> +\item[\field{preserve_resources}]
> +        The driver writes this to let device preserve resources whenever driver
> has demands.
> +        1 - device need to preserve resources which can't be re-created, until 0 is
> set.
> +        0 - all resources can be destroyed.
>  \end{description}
> 
>  \devicenormative{\paragraph}{Common configuration structure layout}{Virtio
> Transport Options / Virtio Over PCI Bus / PCI Device Layout / Common
> configuration structure layout}
> --
> 2.34.1


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