[PATCH] virtio-mem: introduce VIRTIO_MEM_F_PERSISTENT_SUSPEND

[Juraj Marcin <jmarcin@redhat.com>]

Before, the behavior while suspending to a deep sleep state and waking
up was not specified. For example, in x86 QEMU VM all devices receive a
reset request during wake-up. This would lead to unplugging of all the
plugged memory blocks. Due to this, suspending is disallowed in the
Linux Kernel, when plugged memory is present and
VIRTIO_MEM_F_PERSISTENT_SUSPEND feature flag is not advertised by the
virtio-mem device.

This new flag should signal to the guest driver, that the device can
correctly suspend to a deep sleep state and then wake up without
disrupting the plugged memory blocks. This feature flag is already
supported in the Linux Kernel [1] and should be soon also supported in
QEMU [2].

[1]: https://lore.kernel.org/all/20240318120645.105664-1-david@redhat.com/
[2]: https://lore.kernel.org/all/20240806160756.182524-1-jmarcin@redhat.com/

Signed-off-by: Juraj Marcin <jmarcin@redhat.com>
---
 device-types/mem/description.tex | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/device-types/mem/description.tex b/device-types/mem/description.tex
index 0a6d558..5bff24c 100644
--- a/device-types/mem/description.tex
+++ b/device-types/mem/description.tex
@@ -60,6 +60,8 @@ \subsection{Feature bits}\label{sec:Device Types / Memory Device / Feature bits}
 access unplugged memory. \footnote{On platforms with memory properties that
 might get modified implicitly on memory access, this feature is expected to
 be offered by the device.}
+\item[VIRTIO_MEM_F_PERSISTENT_SUSPEND (2)] The driver can allow the guest
+system to enter suspended state (deep sleep, suspend-to-RAM).
 \end{description}
 
 \subsection{Device configuration layout}\label{sec:Device Types / Memory Device / Device configuration layout}
@@ -174,6 +176,9 @@ \subsection{Device Initialization}\label{Device Types / Memory Device / Device I
 The device MUST NOT modify memory or memory properties of plugged memory
 blocks during device reset.
 
+The device SHOULD offer VIRTIO_MEM_F_PERSISTENT_SUSPEND if the platform
+supports suspending (deep sleep, suspend-to-RAM) with plugged memory blocks.
+
 \subsection{Device Operation}\label{sec:Device Types / Memory Device / Device Operation}
 
 The device notifies the driver about the amount of memory the device wants
@@ -268,6 +273,9 @@ \subsection{Device Operation}\label{sec:Device Types / Memory Device / Device Op
 memory blocks to unplug them, or it would not be able to make use of
 unplugged memory blocks after plugging them (e.g., alignment).
 
+If VIRTIO_MEM_F_PERSISTENT_SUSPEND has not been negotiated, the driver MUST NOT
+allow the guest system to enter a suspended state (deep sleep, suspend-to-RAM).
+
 \devicenormative{\subsubsection}{Device Operation}{Device Types / Memory Device / Device Operation}
 
 The device MUST provide the exact same memory properties with the exact same
@@ -293,6 +301,10 @@ \subsection{Device Operation}\label{sec:Device Types / Memory Device / Device Op
 
 The device SHOULD unplug all memory blocks during system resets.
 
+If VIRTIO_MEM_F_PERSISTENT_SUSPEND has been negotiated, the device MUST NOT not
+change the state of memory blocks when suspending or when waking up from
+suspended state (deep sleep, suspend-to-RAM).
+
 \subsubsection{PLUG request}\label{sec:Device Types / Memory Device / Device Operation / PLUG request}
 
 Request to plug consecutive memory blocks that are currently unplugged.
-- 
2.45.2