Re: [RFC PATCH] hw/acpi: do not let OSPM set pcie native hotplug when acpi hotplug is enabled

[Igor Mammedov <imammedo@redhat.com>]

On Mon, 5 Sep 2022 22:25:25 +0530 (IST)
Ani Sinha <ani@anisinha.ca> wrote:

> On Mon, 5 Sep 2022, Ani Sinha wrote:
> 
> >
> >  
> 
> > > >
> > > > diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
> > > > index 0355bd3dda..3dc9379f27 100644
> > > > --- a/hw/i386/acpi-build.c
> > > > +++ b/hw/i386/acpi-build.c
> > > > @@ -1348,10 +1348,12 @@ static Aml *build_q35_osc_method(bool enable_native_pcie_hotplug)
> > > >  {
> > > >      Aml *if_ctx;
> > > >      Aml *if_ctx2;
> > > > +    Aml *if_ctx3;
> > > >      Aml *else_ctx;
> > > >      Aml *method;
> > > >      Aml *a_cwd1 = aml_name("CDW1");
> > > >      Aml *a_ctrl = aml_local(0);
> > > > +    Aml *a_pcie_nhp_ctl = aml_local(1);
> > > >
> > > >      method = aml_method("_OSC", 4, AML_NOTSERIALIZED);
> > > >      aml_append(method, aml_create_dword_field(aml_arg(3), aml_int(0), "CDW1"));
> > > > @@ -1366,11 +1368,26 @@ static Aml *build_q35_osc_method(bool enable_native_pcie_hotplug)
> > > >      /*
> > > >       * Always allow native PME, AER (no dependencies)
> > > >       * Allow SHPC (PCI bridges can have SHPC controller)
> > > > -     * Disable PCIe Native Hot-plug if ACPI PCI Hot-plug is enabled.
> > > >       */
> > > > -    aml_append(if_ctx, aml_and(a_ctrl,
> > > > -        aml_int(0x1E | (enable_native_pcie_hotplug ? 0x1 : 0x0)), a_ctrl));
> > > > +    aml_append(if_ctx, aml_and(a_ctrl, aml_int(0x1F), a_ctrl));  
> > >
> > > that makes us not actually mask any capabilities since you forgot to mask
> > > bit 1 later under if_ctx3 context.
> > >
> > > So OSPM will see a permanent failure (_OSC failure bit in CWD1)
> > > and will have no idea that PCI Hotplug is not supported since we return CWD3
> > > with this bit still set whoever much it tries to negotiate.  
> >
> > The failure is only returned when the OS requests/probes native hotplug
> > capability in CWD1.  
> 
> I meant CWD3.
For OSPM to know which features are denied by platform, the later must mask
them in returned bitmask, how otherwise you would see above behavior.
(or alternatively OSPM might ignore _OSC results and resort to workarounds/probing
as result enabling native hotplug in which case you would see ATS
detected).

To verify which hotplug is used, you can just trace acpi_pci_* in QEMU and observe
if it's used for unplug or not.