[RFC PATCH v6 0/4] virtio: add noirq system sleep PM callbacks for virtio-mmio

[Sungho Bae <baver.bae@gmail.com>]

From: Sungho Bae <baver.bae@lge.com>

Hi all,

Some virtio-mmio based devices, such as virtio-clock or virtio-regulator,
must become operational before other devices have their regular PM restore
callbacks invoked, because those other devices depend on them.

Generally, PM framework provides the three phases (freeze, freeze_late,
freeze_noirq) for the system sleep sequence, and the corresponding resume
phases. But, virtio core only supports the normal freeze/restore phase,
so virtio drivers have no way to participate in the noirq phase, which runs
with IRQs disabled and is guaranteed to run before any normal-phase restore
callbacks.

This series adds the infrastructure and the virtio-mmio transport
wiring so that virtio drivers can implement freeze_noirq/restore_noirq
callbacks.

Design overview
===============

The noirq phase runs with device IRQ handlers disabled and must avoid
sleepable operations. The main constraints addressed are:

 - might_sleep() in virtio_add_status() and virtio_features_ok().
 - virtio_synchronize_cbs() in virtio_reset_device() (IRQs are already
   quiesced).
 - spin_lock_irq() in virtio_config_core_enable() (not safe to call
   with interrupts already disabled).
 - Memory allocation during vq setup (virtqueue_reinit_vring() reuses
   existing buffers instead).

The series provides noirq-safe variants for each of these, plus a new
config_ops->reset_vqs() callback that lets the transport reprogram
queue registers without freeing/reallocating vring memory.

Not all transports can safely perform these operations in the noirq phase.
Transports like virtio-ccw issue channel commands and wait for a completion
interrupt, which will never arrive while device interrupts are masked at
the interrupt controller. A new boolean field config_ops->noirq_safe marks
transports that implement reset/status operations via simple MMIO
reads/writes and are therefore safe to use in noirq context. The noirq
helpers assert this flag at runtime, and virtio_device_freeze_noirq()
enforces it at freeze time, returning -EOPNOTSUPP early to prevent
a deadlock on resume.

When a driver implements restore_noirq, the device bring-up (reset ->
ACKNOWLEDGE -> DRIVER -> finalize_features -> FEATURES_OK) happens in
the noirq phase. The subsequent normal-phase virtio_device_restore()
detects this and skips the redundant re-initialization.

Patch breakdown
===============

Patch 1 is a preparatory refactoring with no functional change.
Patches 2-3 add the core infrastructure. Patch 4 wires it up for
virtio-mmio.

 1. virtio: separate PM restore and reset_done paths

    Splits virtio_device_restore_priv() into independent
    virtio_device_restore() and virtio_device_reset_done() paths,
    using a shared virtio_device_reinit() helper. This is a pure
    refactoring to make the restore path independently extensible
    without complicating the boolean dispatch.

 2. virtio_ring: export virtqueue_reinit_vring() for noirq restore

    Adds virtqueue_reinit_vring(), an exported wrapper that resets
    vring indices and descriptor state in place without any memory
    allocation, making it safe to call from noirq context. Also
    resets IN_ORDER-specific state (free_head, batch_last.id) in
    virtqueue_init() to keep the ring consistent after reinit, and
    adds runtime WARN_ON checks for unexpected in-flight descriptor
    state and split-ring index consistency.

 3. virtio: add noirq system sleep PM infrastructure

    Adds noirq-safe helpers (virtio_add_status_noirq,
    virtio_features_ok_noirq, virtio_reset_device_noirq,
    virtio_config_core_enable_noirq, virtio_device_ready_noirq) and
    the freeze_noirq/restore_noirq driver callbacks plus the
    config_ops->reset_vqs() transport hook. Introduces
    config_ops->noirq_safe to mark transports whose reset/status
    operations are safe in noirq context (e.g. simple MMIO), and
    enforces early -EOPNOTSUPP in virtio_device_freeze_noirq() when
    the transport does not meet noirq requirements. Modifies
    virtio_device_restore() to skip bring-up when restore_noirq
    already ran.

 4. virtio-mmio: wire up noirq system sleep PM callbacks

    Implements vm_reset_vqs() which iterates existing virtqueues,
    reinitializes the vring state via virtqueue_reinit_vring(), and
    reprograms the MMIO queue registers. Adds
    virtio_mmio_freeze_noirq/virtio_mmio_restore_noirq and registers
    them via SET_NOIRQ_SYSTEM_SLEEP_PM_OPS(). Sets .noirq_safe = true
    in virtio_mmio_config_ops to declare that MMIO-based status and
    reset operations are safe during the noirq PM phase.

Testing
=======

Build-tested with arm64 cross-compilation.
(make ARCH=arm64 M=drivers/virtio)

Runtime-tested on an internal virtio-mmio platform with virtio-clock,
confirming that the clock device works well before other devices' normal
restore() callbacks run.

Changes
=======

v6:
  virtio_ring: export virtqueue_reinit_vring() for noirq restore
   - Made virtqueue_reinit_vring() fail with -EBUSY on precondition
     violations and propagate that error.

  virtio: add noirq system sleep PM infrastructure
   - Make noirq restore failure terminal for the same device:
     .restore is not a same-device fallback for .restore_noirq failure.
   - Decouple noirq failure from pre-freeze dev->failed snapshot.
   - Add noirq_safe validation in virtio_device_freeze() to catch transport
     mismatch early.

  virtio-mmio: wire up noirq system sleep PM callbacks
   - Make vm_reset_vqs() handle virtqueue_reinit_vring() failures
     to prevent continuing noirq restore with a potentially corrupted
     split free-list state.

v5:
  virtio: add noirq system sleep PM infrastructure
   - Preserve FAILED across restore_noirq() failure by recording the
     failure in dev->failed before falling back to the normal restore path.
   - Document the restore/restore_noirq fallback contract more clearly,
     especially for drivers that preserve virtqueues across suspend.

v4:
  virtio_ring: export virtqueue_reinit_vring() for noirq restore
   - Reinit safety was tightened by resetting IN_ORDER-specific state.
   - Added extra split-ring consistency WARN_ON checks.
   - Clarified caller preconditions for noirq-safe vring reinit.

  virtio: add noirq system sleep PM infrastructure
   - Added config_ops->noirq_safe to explicitly mark transports that are
     safe in noirq PM phase.
   - Enforced early -EOPNOTSUPP checks in freeze_noirq for unsupported
     transport combinations (noirq_safe/reset_vqs requirements).
   - Added defensive runtime guards/warnings in noirq helper and restore
     paths.
   - Discussed the freeze-before-freeze_noirq abort scenario raised in
     review and concluded that the fallback restore path is intentionally
     handled by regular restore after core reinit/reset, so reset_vqs is
     kept limited to the noirq restore flow.

  virtio-mmio: wire up noirq system sleep PM callbacks
   - Marked virtio-mmio transport as noirq-capable by setting .noirq_safe
     in virtio_mmio_config_ops.

v3:
  virtio: separate PM restore and reset_done paths
   - Refined restore flow to explicitly handle the no-driver case after
     reinit, improving clarity and avoiding unnecessary driver-path
     assumptions.

  virtio_ring: export virtqueue_reinit_vring() for noirq restore
   - Hardened virtqueue_reinit_vring() with stronger safety notes and
     a runtime WARN_ON check to catch reinit with unexpected free-list
     state.

  virtio: add noirq system sleep PM infrastructure
   - Added explicit noirq restore completion tracking noirq_restore_done
     and updated PM sequencing to use it, plus early freeze_noirq
     validation for missing reset_vqs support.

  virtio-mmio: wire up noirq system sleep PM callbacks
   - Updated virtio-mmio restore path to skip legacy GUEST_PAGE_SIZE
     rewrite when noirq restore already completed.

v2:
  virtio-mmio: wire up noirq system sleep PM callbacks
   - The code that was duplicated in vm_setup_vq() and vm_reset_vqs() has
     been moved to vm_active_vq() function.


Sungho Bae (4):
  virtio: separate PM restore and reset_done paths
  virtio_ring: export virtqueue_reinit_vring() for noirq restore
  virtio: add noirq system sleep PM infrastructure
  virtio-mmio: wire up noirq system sleep PM callbacks

 drivers/virtio/virtio.c       | 318 +++++++++++++++++++++++++++++++---
 drivers/virtio/virtio_mmio.c  | 136 ++++++++++-----
 drivers/virtio/virtio_ring.c  |  58 +++++++
 include/linux/virtio.h        |  24 +++
 include/linux/virtio_config.h |  39 +++++
 include/linux/virtio_ring.h   |   3 +
 6 files changed, 512 insertions(+), 66 deletions(-)

-- 
2.43.0