[virtio-comment] Re: [VIRTIO PCI PATCH v5 1/1] transport-pci: Add freeze_mode to virtio_pci_common_cfg

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Sep 19, 2023 at 07:42:42PM +0800, Jiqian Chen wrote:
> When guest vm does S3, Qemu will reset and clear some things of virtio
> devices, but guest can't aware that, so that may cause some problems.
> For excample, Qemu calls virtio_reset->virtio_gpu_gl_reset when guest
> resume, that function will destroy render resources of virtio-gpu. As
> a result, after guest resume, the display can't come back and we only
> saw a black screen. Due to guest can't re-create all the resources, so
> we need to let Qemu not to destroy them when S3.
> 
> For above purpose, we need a mechanism that allows guests and QEMU to
> negotiate their reset behavior. So this patch add a new parameter
> named freeze_mode to struct virtio_pci_common_cfg. And when guest
> suspends, it can write freeze_mode to be FREEZE_S3, and then virtio
> devices can change their reset behavior on Qemu side according to
> freeze_mode. What's more, freeze_mode can be used for all virtio
> devices to affect the behavior of Qemu, not just virtio gpu device.
> 
> Signed-off-by: Jiqian Chen <Jiqian.Chen@amd.com>
> ---
>  transport-pci.tex | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/transport-pci.tex b/transport-pci.tex
> index a5c6719..2543536 100644
> --- a/transport-pci.tex
> +++ b/transport-pci.tex
> @@ -319,6 +319,7 @@ \subsubsection{Common configuration structure layout}\label{sec:Virtio Transport
>          le64 queue_desc;                /* read-write */
>          le64 queue_driver;              /* read-write */
>          le64 queue_device;              /* read-write */
> +        le16 freeze_mode;               /* read-write */
>          le16 queue_notif_config_data;   /* read-only for driver */
>          le16 queue_reset;               /* read-write */
>

we can't add fields in the middle of the structure like this -
offset of queue_notif_config_data and queue_reset changes.

  
> @@ -393,6 +394,12 @@ \subsubsection{Common configuration structure layout}\label{sec:Virtio Transport
>  \item[\field{queue_device}]
>          The driver writes the physical address of Device Area here.  See section \ref{sec:Basic Facilities of a Virtio Device / Virtqueues}.
>  
> +\item[\field{freeze_mode}]
> +        The driver writes this to set the freeze mode of virtio pci.
> +        VIRTIO_PCI_FREEZE_MODE_UNFREEZE - virtio-pci is running;
> +        VIRTIO_PCI_FREEZE_MODE_FREEZE_S3 - guest vm is doing S3, and virtio-pci enters S3 suspension;
> +        Other values are reserved for future use, like S4, etc.
> +

we need to specify these values then.

we also need
- feature bit to detect support for S3
- conformance statements documenting behavious under S3


>  \item[\field{queue_notif_config_data}]
>          This field exists only if VIRTIO_F_NOTIF_CONFIG_DATA has been negotiated.
>          The driver will use this value when driver sends available buffer
> -- 
> 2.34.1


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