Re: [PATCH v2] acpi: pcihp: make pending delete blocking action expire

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

On Wed, Apr 05, 2023 at 10:34:44AM +0200, Igor Mammedov wrote:
> with Q35 using ACPI PCI hotplug by default, user's request to unplug
> device is ignored when it's issued before guest OS has been booted.
> And any additional attempt to request device hot-unplug afterwards
> results in following error:
> 
>   "Device XYZ is already in the process of unplug"
> 
> arguably it can be considered as a regression introduced by [2],
> before which it was possible to issue unplug request multiple
> times.
> 
> Allowing pending delete blocking expire brings ACPI PCI hotplug
> on par with native PCIe unplug behavior [1] and allows user
> to repeat unplug requests at propper times.
> Set expire timeout to arbitrary 1msec so user won't be able to
> flood guest with SCI interrupts by calling device_del in tight loop.
> 
> PS:
> ACPI spec doesn't mandate what OSPM can do with GPEx.status
> bits set before it's booted => it's impl. depended.
> Status bits may be retained (I tested with one Windows version)
> or cleared (Linux since 2.6 kernel times) during guest's ACPI
> subsystem initialization.
> Clearing status bits (though not wrong per se) hides the unplug
> event from guest, and it's upto user to repeat device_del later
> when guest is able to handle unplug requests.
> 
> 1) 18416c62e3 ("pcie: expire pending delete")
> 2)
> Fixes: cce8944cc9ef ("qdev-monitor: Forbid repeated device_del")
> Signed-off-by: Igor Mammedov <imammedo@redhat.com>

I feel a real solution is to detect guest handling the
event such as clearing GPE and allowing resending
the interrupt then.
A similar strategy should be possible with the attention
button.

This patch is more of a band-aid - it is possible that guest
rebooted and so user knows a new device_del is required,
and we arbitrarily reject that. Right?

This is arguably a regression but not in this release yes?
So I don't think it needs to block qemu release.


> ---
> v2:
>    * change timeout to 1ms
>    * add comment to expire usage
>    * massage commit message to be a bit more clear
> 
> CC: mst@redhat.com
> CC: anisinha@redhat.com
> CC: jusual@redhat.com
> CC: kraxel@redhat.com

It's helpful to have CC before --- so backporters know whom to CC, too.

> ---
>  hw/acpi/pcihp.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/hw/acpi/pcihp.c b/hw/acpi/pcihp.c
> index dcfb779a7a..5daa732a33 100644
> --- a/hw/acpi/pcihp.c
> +++ b/hw/acpi/pcihp.c
> @@ -357,6 +357,16 @@ void acpi_pcihp_device_unplug_request_cb(HotplugHandler *hotplug_dev,
>       * acpi_pcihp_eject_slot() when the operation is completed.
>       */
>      pdev->qdev.pending_deleted_event = true;
> +    /* if unplug was requested before OSPM is initialized,
> +     * linux kernel will clear GPE0.sts[] bits during boot, which effectively
> +     * hides unplug event. BAnd than followup qmp_device_del() calls remain

BAnd?

> +     * blocked by above flag permanently.
> +     * Unblock qmp_device_del() by setting expire limit, so user can
> +     * repeat unplug request later when OSPM has been booted.
> +     */
> +    pdev->qdev.pending_deleted_expires_ms =
> +        qemu_clock_get_ms(QEMU_CLOCK_VIRTUAL); /* 1 msec */
> +
>      s->acpi_pcihp_pci_status[bsel].down |= (1U << slot);
>      acpi_send_event(DEVICE(hotplug_dev), ACPI_PCI_HOTPLUG_STATUS);
>  }
> -- 
> 2.39.1