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

[Ani Sinha]

On Mon, 5 Sep 2022, Igor Mammedov wrote:

> On Mon,  5 Sep 2022 12:55:31 +0530
> Ani Sinha <ani@anisinha.ca> wrote:
>
> > Possible fix for https://bugzilla.redhat.com/show_bug.cgi?id=2089545
> >
> > Change in AML:
> >
> > @@ -47,33 +47,39 @@
> >      Scope (_SB)
> >      {
> >          Device (PCI0)
> >          {
> >              Name (_HID, EisaId ("PNP0A08") /* PCI Express Bus */)  // 
> > _HID: Hardware ID
> >              Name (_CID, EisaId ("PNP0A03") /* PCI Bus */)  // _CID: 
> > Compatible ID
> >              Name (_ADR, Zero)  // _ADR: Address
> >              Name (_UID, Zero)  // _UID: Unique ID
> >              Method (_OSC, 4, NotSerialized)  // _OSC: Operating System 
> > Capabilities
> >              {
> >                  CreateDWordField (Arg3, Zero, CDW1)
> >                  If ((Arg0 == ToUUID 
> > ("33db4d5b-1ff7-401c-9657-7441c03dd766") /* PCI Host Bridge Device */))
> >                  {
> >                      CreateDWordField (Arg3, 0x04, CDW2)
> >                      CreateDWordField (Arg3, 0x08, CDW3)
> >                      Local0 = CDW3 /* \_SB_.PCI0._OSC.CDW3 */
> > -                    Local0 &= 0x1E
> > +                    Local0 &= 0x1F
> > +                    Local1 = (CDW3 & One)
> > +                    If ((One == Local1))
> > +                    {
> > +                        CDW1 |= 0x12
> > +                    }
> > +
> >                      If ((Arg1 != One))
> >                      {
> >                          CDW1 |= 0x08
> >                      }
> >
> >                      If ((CDW3 != Local0))
> >                      {
> >                          CDW1 |= 0x10
> >                      }
> >
> >                      CDW3 = Local0
> >                  }
> >                  Else
> >                  {
> >                      CDW1 |= 0x04
> >                  }
> > **
> >
> > Signed-off-by: Ani Sinha <ani@anisinha.ca>
> > ---
> >  hw/i386/acpi-build.c | 23 ++++++++++++++++++++---
> >  1 file changed, 20 insertions(+), 3 deletions(-)
> >
> > 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.

Also maybe I was mistaken but in the offline thread, I thought MST
suggested we left the native hotplug bit as is without ever masking it.
I guess he was coming from the fact that experiments indicated that when
the native hotplug is off in OSC, Windows did not detect ATS.


> So if it boots at all, guest will probably not use any requested features
> since _OSC failed to confirm any without error.
>
> And even if we clear hotplug bit it still doesn't help.
>
> some more testing shows that ATS cap doesn't depended hard on native hotplug
> (i.e. run QEMU with native hotplug enabled but disable hotplug on root-port 
> in question)
> To me it still looks like a bug in Windows' acpi hotplug impl
> (or perhaps it's no more properly maintained, so it doesn't account for new 
> features).

Yeah I also think this is a Windows bug. ATS and native hotplug does not
seem to be related in anyway.

>
> > +    /*
> > +     * if ACPI PCI Hot-plug is enabled, do not let OSPM set OSC PCIE
> > +     * Native hotplug ctrl bit.
> > +     */
> > +    if (!enable_native_pcie_hotplug) {
> > +        /* check if the ACPI native hotplug bit is set by the OS in DWORD3 
> > */
> > +        aml_append(if_ctx, aml_and(aml_name("CDW3"),
> > +                                   aml_int(0x01), a_pcie_nhp_ctl));
> > +        if_ctx3 = aml_if(aml_equal(aml_int(1), a_pcie_nhp_ctl));
> > +        /*
> > +         * ACPI spec 5.1, section 6.2.11
> > +         * bit 1 in first DWORD - _OSC failure
> > +         * bit 4 in first DWORD - capabilities masked
> > +         */
> > +        aml_append(if_ctx3, aml_or(a_cwd1, aml_int(0x12), a_cwd1));
> > +        aml_append(if_ctx, if_ctx3);
> > +    }
> >      if_ctx2 = aml_if(aml_lnot(aml_equal(aml_arg(1), aml_int(1))));
> >      /* Unknown revision */
> >      aml_append(if_ctx2, aml_or(a_cwd1, aml_int(0x08), a_cwd1));
>
>