Re: [RFC PATCH v3 0/7] Use ACPI PCI hot-plug for Q35

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

On Thu, Sep 24, 2020 at 09:00:06AM +0200, Julia Suvorova wrote:
> The patch set consists of two parts:
> patches 1-4: introduce new feature
>              'acpi-pci-hotplug-with-bridge-support' on Q35
> patches 5-7: make the feature default along with changes in ACPI tables
> 
> This way maintainers can decide which way to choose without breaking
> the patch set.
> 
> With the feature disabled Q35 falls back to the native hot-plug.
> 
> Pros
>     * no racy behavior during boot (see 110c477c2ed)
>     * eject is possible - according to PCIe spec, attention button
>       press should lead to power off, and then the adapter should be
>       removed manually. As there is no power down state exists in QEMU,
>       we cannot distinguish between an eject and a power down
>       request.
>     * no delay during deleting - after the actual power off software
>       must wait at least 1 second before indicating about it. This case
>       is quite important for users, it even has its own bug:
>           https://bugzilla.redhat.com/show_bug.cgi?id=1594168
>     * no timer-based behavior - in addition to the previous example,
>       the attention button has a 5-second waiting period, during which
>       the operation can be canceled with a second press. While this
>       looks fine for manual button control, automation will result in
>       the need to queue or drop events, and the software receiving
>       events in all sort of unspecified combinations of attention/power
>       indicator states, which is racy and uppredictable.
>     * fixes:
>         * https://bugzilla.redhat.com/show_bug.cgi?id=1752465
>         * https://bugzilla.redhat.com/show_bug.cgi?id=1690256
> 
> Cons:
>     * lose per-port control over hot-plug (can be resolved)
>     * no access to possible features presented in slot capabilities
>       (this is only surprise removal AFAIK)

something I don't quite get is whether with this one can still add
new pcie bridges and then hotplug into these with native
hotplug.

the challenge to answering this with certainty is that right now qemu
does not allow hotplugging complex devices such as bridges at all,
we only have a hack for multifunction devices.
Maybe adding a bridge as function 1 on command line, then hotplugging another device as
function 0 will work to test this.



> v3:
>     * drop change of _OSC to allow SHPC on hotplugged bridges
>     * use 'acpi-root-pci-hotplug'
>     * add migration states [Igor]
>     * minor style changes
> 
> v2:
>     * new ioport range for acpiphp [Gerd]
>     * drop find_pci_host() [Igor]
>     * explain magic numbers in _OSC [Igor]
>     * drop build_q35_pci_hotplug() wrapper [Igor]
> 
> Julia Suvorova (7):
>   hw/acpi/pcihp: Enhance acpi_pcihp_disable_root_bus() to support Q35
>   hw/i386/acpi-build: Add ACPI PCI hot-plug methods to Q35
>   hw/pci/pcie: Do not initialize slot capability if acpihp is used
>   hw/acpi/ich9: Enable ACPI PCI hot-plug
>   bios-tables-test: Allow changes in DSDT ACPI tables
>   hw/acpi/ich9: Set ACPI PCI hot-plug as default
>   bios-tables-test: Update golden binaries
> 
>  hw/i386/acpi-build.h              |   7 ++++
>  include/hw/acpi/ich9.h            |   5 +++
>  include/hw/acpi/pcihp.h           |   3 +-
>  hw/acpi/ich9.c                    |  67 ++++++++++++++++++++++++++++++
>  hw/acpi/pcihp.c                   |  16 ++++---
>  hw/acpi/piix4.c                   |   4 +-
>  hw/i386/acpi-build.c              |  31 ++++++++------
>  hw/i386/pc.c                      |   1 +
>  hw/pci/pcie.c                     |  16 +++++++
>  tests/data/acpi/q35/DSDT          | Bin 7678 -> 7950 bytes
>  tests/data/acpi/q35/DSDT.acpihmat | Bin 9002 -> 9274 bytes
>  tests/data/acpi/q35/DSDT.bridge   | Bin 7695 -> 9865 bytes
>  tests/data/acpi/q35/DSDT.cphp     | Bin 8141 -> 8413 bytes
>  tests/data/acpi/q35/DSDT.dimmpxm  | Bin 9331 -> 9603 bytes
>  tests/data/acpi/q35/DSDT.ipmibt   | Bin 7753 -> 8025 bytes
>  tests/data/acpi/q35/DSDT.memhp    | Bin 9037 -> 9309 bytes
>  tests/data/acpi/q35/DSDT.mmio64   | Bin 8808 -> 9080 bytes
>  tests/data/acpi/q35/DSDT.numamem  | Bin 7684 -> 7956 bytes
>  tests/data/acpi/q35/DSDT.tis      | Bin 8283 -> 8555 bytes
>  19 files changed, 129 insertions(+), 21 deletions(-)
> 
> -- 
> 2.25.4