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

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

On Thu, Oct 01, 2020 at 10:55:35AM +0200, Julia Suvorova wrote:
> On Thu, Sep 24, 2020 at 11:20 AM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > 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.
> 
> Right now I disable native if there is acpihp anywhere, but even if
> you enable it for hotplugged devices, native hot-plug will not work.

So that's a minor regression in functionality, right?
Why is that the case? Because you disable it in ACPI?
What if we don't?

> > 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
> >