Re: [PATCH RFC v4 0/5] PCI: pciehp: Report surprise removal during safe removal

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

On Mon, Oct 12, 2026 at 07:39:34AM +0200, Abhin Parekadan Jose wrote:
> Gentle ping on this RFC.
> 
> Lukas, Bjorn, could you let me know whether the general direction is
> acceptable before I work on a non-RFC version? Any comments on the
> approach would help.
> 
> Thanks,
> Abhin

I'd say, complete the picture. acpi,shpc,polling...

Maybe think about how this interacts with pci_ignore_hotplug
(seems to weirdly check presence then ignore)?

And test with virtio not with edu.

> On Sun, Sep 27, 2026 at 06:20:11PM +0000, Abhin Parekadan Jose wrote:
> > If a safe removal is already in progress when the device is surprise
> > removed, pciehp cannot report the disconnect [1]. The removal blocks
> > waiting on a device interrupt or status read, and pciehp's IRQ thread is
> > single-threaded and is itself executing that removal, so it never runs
> > again to report the device gone. The removal hangs indefinitely.
> > 
> > pciehp_isr() does run while the IRQ thread is blocked, but pciehp_ist()
> > must ignore link and presence changes caused by Secondary Bus Reset or
> > DPC, and telling those apart takes seconds which cannot be spent in
> > hardirq.
> > 
> > Patch 4 does not do that work in hardirq. pciehp_isr() only checks
> > whether the card is present or the link is active, and defers the rest
> > to a work item in process context. The work item does not need to know
> > why the link changed, only whether the card is gone: it waits for DPC
> > recovery or a Secondary Bus Reset in progress to complete, then checks
> > again under reset_lock, and if the card is absent and the link is down
> > marks the devices below disconnected, which schedules the driver's
> > disconnect work from patch 1. pciehp_ist() is unchanged and
> > remains the only consumer of the one-shot flags PCI_DPC_RECOVERED and
> > PCI_LINK_CHANGED, which tell it whether a link change can be ignored.
> > 
> > Patches:
> > 
> >   1/5 Michael's "PCI: Report surprise removal event" from his RFC v5,
> >       which adds disconnect_work_enable and pdev->disconnect_work.
> >       Changed to serialize disconnect_work_enable with a per-device
> >       spinlock.
> > 
> >   2/5 Add pci_hp_wait_link_change(), which awaits a Secondary Bus Reset
> >       in progress without consuming PCI_LINK_CHANGED.
> > 
> >   3/5 Add pci_dpc_wait_recovery(), which awaits DPC recovery without
> >       consuming PCI_DPC_RECOVERED.
> > 
> >   4/5 The pciehp change. Adds disconnect_work to struct controller,
> >       scheduled from pciehp_isr() on PDC or DLLSC when neither Presence
> >       Detect State nor Data Link Layer Link Active indicates a card.


how about including the virtio patch? seems more relevant than
fooling around with edu.

> >   5/5 A POC driver for the QEMU edu device that blocks in remove()
> >       waiting for an interrupt, standing in for del_gendisk() stuck in
> >       blk_mq_freeze_queue_wait(). Not for merge -- included so the
> >       hang can be reproduced.
> > 
> > Changes since RFC v3 [5]:
> > 
> >   - 5/5: Reinitialize the completion in remove() before requesting the
> >     delayed interrupt, so that an earlier completion cannot let remove()
> >     return while the interrupt is still pending. (Sashiko)
> >   - 1/5 to 4/5: No changes.
> > 
> > Changes since RFC v2 [4]:
> > 
> >   - 1/5: Protect disconnect_work_enable with a per-device spinlock, held
> >     while testing it and scheduling the work, instead of lockless
> >     accesses and barriers.  pci_dev_set_disconnected() could otherwise
> >     test the flag, get preempted, and queue the work while a newly bound
> >     driver re-initializes it.  With the lock, cancel_work_sync() is
> >     sufficient again, so go back to it from disable_work_sync().
> >     (Sashiko)
> >   - 2/5: Test PCI_LINK_CHANGING with test_bit_acquire(), so that the
> >     caller's subsequent accesses are ordered after the end of the code
> >     section even if wait_event() returns without sleeping. (Sashiko)
> >   - 3/5, 4/5, 5/5: No changes.
> > 
> > Changes since RFC v1 [2]:
> > 
> >   - 1/5: Use disable_work_sync() instead of cancel_work_sync() in
> >     pci_clear_disconnect_work(), so that a racing schedule_work() cannot
> >     queue the work after remove() has returned. (Sashiko)
> >   - 2/5, 3/5: New.
> >   - 4/5: Drop schedule_notification_work(); pci_dev_set_disconnected()
> >     schedules the driver's disconnect work for all callers again.
> >     (Michael)
> >   - 4/5: Don't consume PCI_DPC_RECOVERED and PCI_LINK_CHANGED in the
> >     work item.  In v1 it ran the same spurious link change test as
> >     pciehp_ist(), and whichever ran first took the flags, so pciehp_ist()
> >     could tear down a device that was only reset. (Sashiko)
> >   - 4/5: Check presence with pciehp_card_present_or_link_active() in
> >     both pciehp_isr() and the work item, as pciehp_ist() does, so that a
> >     port with Presence Detect State hardwired to zero is not mistaken
> >     for an empty slot.
> >   - 4/5: Return early from the work item if pciehp_ist() has already
> >     taken the pending events, and treat a read error of the presence
> >     check as "not present". (Sashiko)
> >   - 4/5: In pciehp_isr(), check presence before dropping the runtime PM
> >     reference on the port's parent.  In the work item, take a runtime PM
> >     reference and check presence under reset_lock, since a slot reset
> >     may make Presence Detect State and Link Active flap.
> >   - 5/5: Build only with CONFIG_EDU_SRPOC, which depends on PCI; don't
> >     claim the interrupt when the device reads all ones; clear bus
> >     mastering on teardown. (Sashiko)
> > 
> > Testing
> > 
> > Reproducing this needs QEMU changes, since neither device_del nor the
> > attention button produces a true surprise removal.  A branch with both
> > is here [3]:
> > 
> >   - a delayed-IRQ register on the edu device (BAR0 0x30, write N ms)
> >   - a pcie_surprise_del monitor command that drops the device and
> >     generates PDC=1, DLLSC=1, PDS=0
> > 
> > Both tests use:
> > 
> >   ./qemu-system-aarch64 -machine virt,gic-version=3 -cpu cortex-a57 \
> >     -m 512 -smp 2 -kernel Image -initrd initramfs.cpio.gz \
> >     -device pcie-root-port,id=rp1,chassis=1,slot=1 \
> >     -device edu,bus=rp1,id=edu0 -append "console=ttyAMA0 rdinit=/init" \
> >     -nographic -monitor unix:/tmp/qemu-mon.sock,server,nowait
> > 
> >   and need a guest kernel with CONFIG_EDU_SRPOC=y.
> > 
> > Test 1 (Hang in remove() on a user thread, then surprise removal):
> > 
> >   guest# echo 1 > /sys/bus/pci/devices/0000:01:00.0/remove &
> >   host$  echo "pcie_surprise_del edu0" | socat - unix-connect:/tmp/qemu-mon.sock
> > 
> >   This is the case patch 1 solves on its own: pciehp's IRQ thread is
> >   free and handles the removal.
> > 
> > Test 2 (Hang in remove() on pciehp's IRQ thread, then surprise removal):
> > 
> >   guest# echo 0 > /sys/bus/pci/slots/1/power &
> >   host$  echo "pcie_surprise_del edu0" | socat - unix-connect:/tmp/qemu-mon.sock
> > 
> >   Without patch 4 the safe removal does not return until the delayed
> >   interrupt fires 600 s later.  With it,
> >   pciehp_isr() schedules ctrl->disconnect_work, which marks the device
> >   disconnected; the POC driver's disconnect work completes the wait and
> >   remove() proceeds.
> > 
> > Open questions
> > 
> >   - Is this a viable approach?
> > 
> > [1] https://lore.kernel.org/all/aHlZE18kPuHuDtTT@wunner.de/
> > [2] https://lore.kernel.org/all/20260905183905.997833-1-abhinjoses@gmail.com/
> > [3] https://gitlab.com/abhinkop/qemu/-/commits/suprise-removal
> > [4] https://lore.kernel.org/all/20260927165459.829900-1-abhinjoses@gmail.com/
> > [5] https://lore.kernel.org/all/20260927175203.928270-1-abhinjoses@gmail.com/
> > 
> > Assisted-by: LLM
> > 
> > Abhin Parekadan Jose (4):
> >   PCI: pciehp: Add pci_hp_wait_link_change()
> >   PCI/DPC: Add pci_dpc_wait_recovery()
> >   PCI: pciehp: Report surprise removal from pciehp_isr()
> >   misc: Add edu_srpoc surprise removal POC driver
> > 
> > Michael S. Tsirkin (1):
> >   PCI: Report surprise removal event
> > 
> >  drivers/misc/Kconfig                   |  11 ++
> >  drivers/misc/Makefile                  |   1 +
> >  drivers/misc/edu_srpoc.c               | 172 +++++++++++++++++++++++++
> >  drivers/pci/hotplug/pci_hotplug_core.c |  21 ++-
> >  drivers/pci/hotplug/pciehp.h           |   1 +
> >  drivers/pci/hotplug/pciehp_hpc.c       |  67 ++++++++++
> >  drivers/pci/pci.h                      |  10 ++
> >  drivers/pci/pcie/dpc.c                 |  54 ++++++--
> >  drivers/pci/probe.c                    |   1 +
> >  include/linux/pci.h                    |  57 ++++++++
> >  10 files changed, 381 insertions(+), 14 deletions(-)
> >  create mode 100644 drivers/misc/edu_srpoc.c
> > 
> > 
> > base-commit: fd179f8a05be3ccae366b9b96e176b51fbe54aab
> > --
> > 2.51.1
> >