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

From: Zhu, Lingshan
Date: Wed Sep 20 2023 - 03:43:06 EST



On 9/20/2023 3:17 PM, Chen, Jiqian wrote:
Hi Lingshan,

On 2023/9/20 14:58, Zhu, Lingshan wrote:

On 9/20/2023 2:33 PM, Chen, Jiqian wrote:
Hi Lingshan,

On 2023/9/20 13:59, Zhu, Lingshan wrote:
On 9/19/2023 8:31 PM, Michael S. Tsirkin wrote:
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.
Hi Jiqian,

Have you seen this series: [PATCH 0/5] virtio: introduce SUSPEND bit and vq state
https://lore.kernel.org/all/3f4cbf84-010c-cffa-0b70-33c449b5561b@xxxxxxxxx/T/

We introduced a bit in the device status SUSPEND, when VIRTIO_F_SUSPEND is
negotiated, the driver can set SUSPEND in the device status to suspend the
device.

When SUSPEND, the device should pause its operations and preserve its configurations
in its configuration space.

The driver re-write DRIVER_OK to clear SUSPEND, so the device resumes running.

This is originally to serve live migration, but I think it can also meet your needs.
I noticed your series, but I am not sure they are also meet my needs.
If driver write 0 to reset device, can the SUSPEND bit be cleared? (pci_pm_resume->virtio_pci_restore->virtio_device_restore->virtio_reset_device)
if the driver writes 0, it resets all virtio functionalities. So SUSPEND is cleared.
Then your patches are not meet my needs. In my scene, it needs to keep the SUSPEND bit util the resume process is complete.
Because in my virtio-gpu scene, when guest resume, it call virtio_reset_device to clear all device status bits, and then reset virtio-gpu in Qemu, and then destroy render resources, I don't want the resources are destroyed during the resume process. So, I add freeze_mode to tell Qemu that guest is doing S3 and resources need to be kept.
When a guest set to S3, the hypervisor suspend the guest to RAM, and the passthrough-ed device are in low power state.
I am not sure the device can keep its status/context/data, maybe need to recover from RAM anyway.

So I suggest not reset the device, there need some code changes in QEMU
When set to S3, SUSPEND the device, then suspend to RAM
When resume from S3, restore from RAM and set DRIVER_OK to resume running.

device reset can also be used to recover the device from fatal errors, so it should reset everything in virtio.
If SUSPEND is cleared, then during the reset process in Qemu, I can't judge if the reset request is from guest restore process or not, and then I can't change the reset behavior.
I think when enter S3, the hypervisor/driver should set SUSPEND to the device. And when resume from S3, the hypervisor/driver should
re-write DRIVER_OK to clear SUSPEND, then the device resume running.
Can you send me your patch link on kernel and qemu side? I will take a deep look.
There are no patches for qemu/kernel yet, spec first.
Thanks,
Zhu Lingshan
Signed-off-by: Jiqian Chen <Jiqian.Chen@xxxxxxx>
---
   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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