RE: [PATCH RFC V2 35/37] hw/arm: Support hotplug capability check using _OSC method

[Salil Mehta via <qemu-devel@nongnu.org>]

Hi Gavin,

> From: Gavin Shan <gshan@redhat.com>
> Sent: Friday, September 29, 2023 5:23 AM
> To: Salil Mehta <salil.mehta@huawei.com>; qemu-devel@nongnu.org; qemu-arm@nongnu.org
> Cc: maz@kernel.org; jean-philippe@linaro.org; Jonathan Cameron
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> zhukeqian <zhukeqian1@huawei.com>; wangxiongfeng (C)
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> Subject: Re: [PATCH RFC V2 35/37] hw/arm: Support hotplug capability check
> using _OSC method
> 
> Hi Salil,
> 
> On 9/26/23 20:36, Salil Mehta wrote:
> > Physical CPU hotplug results in (un)setting of ACPI _STA.Present bit. AARCH64
> > platforms do not support physical CPU hotplug. Virtual CPU Hotplug support being
> > implemented toggles ACPI _STA.Enabled Bit to achieve Hotplug functionality. This
> > is not same as physical CPU hotplug support.
> >
> > In future, if ARM architecture supports physical CPU hotplug then the current
> > design of virtual CPU hotplug can be used unchanged. Hence, there is a need for
> > firmware/VMM/Qemu to support evaluation of platform wide capabilitiy related to
> > the *type* of CPU hotplug support present on the platform. OSPM might need this
> > during boot time to correctly initialize the CPUs and other related components
> > in the kernel.
> >
> > NOTE: This implementation will be improved to add the support of *query* in the
> > subsequent versions. This is very minimal support to assist kernel.
> >
> > ASL for the implemented _OSC method:
> >
> > Method (_OSC, 4, NotSerialized)  // _OSC: Operating System Capabilities
> > {
> >      CreateDWordField (Arg3, Zero, CDW1)
> >      If ((Arg0 == ToUUID ("0811b06e-4a27-44f9-8d60-3cbbc22e7b48") /* Platform-wide Capabilities */))
> >      {
> >          CreateDWordField (Arg3, 0x04, CDW2)
> >          Local0 = CDW2 /* \_SB_._OSC.CDW2 */
> >          If ((Arg1 != One))
> >          {
> >              CDW1 |= 0x08
> >          }
> >
> >          Local0 &= 0x00800000
> >          If ((CDW2 != Local0))
> >          {
> >              CDW1 |= 0x10
> >          }
> >
> >          CDW2 = Local0
> >      }
> >      Else
> >      {
> >          CDW1 |= 0x04
> >      }
> >
> >      Return (Arg3)
> > }
> >
> > Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
> > ---
> >   hw/arm/virt-acpi-build.c | 52 ++++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 52 insertions(+)
> >
> > diff --git a/hw/arm/virt-acpi-build.c b/hw/arm/virt-acpi-build.c
> > index cbccd2ca2d..377450dd16 100644
> > --- a/hw/arm/virt-acpi-build.c
> > +++ b/hw/arm/virt-acpi-build.c
> > @@ -861,6 +861,55 @@ static void build_fadt_rev6(GArray *table_data, BIOSLinker *linker,
> >       build_fadt(table_data, linker, &fadt, vms->oem_id, vms-
> >oem_table_id);
> >   }
> >
> > +static void build_virt_osc_method(Aml *scope, VirtMachineState *vms)
> > +{
> > +    Aml *if_uuid, *else_uuid, *if_rev, *if_caps_masked, *method;
> > +    Aml *a_cdw1 = aml_name("CDW1");
> > +    Aml *a_cdw2 = aml_local(0);
> > +
> > +    method = aml_method("_OSC", 4, AML_NOTSERIALIZED);
> > +    aml_append(method, aml_create_dword_field(aml_arg(3), aml_int(0), "CDW1"));
> > +
> > +    /* match UUID */
> > +    if_uuid = aml_if(aml_equal(
> > +        aml_arg(0), aml_touuid("0811B06E-4A27-44F9-8D60-3CBBC22E7B48")));
> > +
> > +    aml_append(if_uuid, aml_create_dword_field(aml_arg(3), aml_int(4), "CDW2"));
> > +    aml_append(if_uuid, aml_store(aml_name("CDW2"), a_cdw2));
> > +
> > +    /* check unknown revision in arg(1) */
> > +    if_rev = aml_if(aml_lnot(aml_equal(aml_arg(1), aml_int(1))));
> > +    /* set revision error bits,  DWORD1 Bit[3] */
> > +    aml_append(if_rev, aml_or(a_cdw1, aml_int(0x08), a_cdw1));
> > +    aml_append(if_uuid, if_rev);
> > +
> > +    /*
> > +     * check support for vCPU hotplug type(=enabled) platform-wide capability
> > +     * in DWORD2 as sepcified in the below ACPI Specification ECR,
> > +     *  # https://bugzilla.tianocore.org/show_bug.cgi?id=4481
> > +     */
> > +    if (vms->acpi_dev) {
> > +        aml_append(if_uuid, aml_and(a_cdw2, aml_int(0x800000), a_cdw2));
> > +        /* check if OSPM specified hotplug capability bits were masked */
> > +        if_caps_masked = aml_if(aml_lnot(aml_equal(aml_name("CDW2"), a_cdw2)));
> > +        aml_append(if_caps_masked, aml_or(a_cdw1, aml_int(0x10), a_cdw1));
> > +        aml_append(if_uuid, if_caps_masked);
> > +    }
> > +    aml_append(if_uuid, aml_store(a_cdw2, aml_name("CDW2")));
> > +
> > +    aml_append(method, if_uuid);
> > +    else_uuid = aml_else();
> > +
> > +    /* set unrecognized UUID error bits, DWORD1 Bit[2] */
> > +    aml_append(else_uuid, aml_or(a_cdw1, aml_int(4), a_cdw1));
> > +    aml_append(method, else_uuid);
> > +
> > +    aml_append(method, aml_return(aml_arg(3)));
> > +    aml_append(scope, method);
> > +
> > +    return;
> > +}
> > +
> 
> The check on vms->acpi_dev seems not enough. We may still need to check
> mc->has_hotpluggable_cpus and vms->gic_version etc. Besides, the "return"
> at end of the function isn't needed.

Agreed. We just need to check 'mc->has_hotpluggable_cpus'. It will cover
everything.

A legacy copy and paste mistake everywhere. Thanks for pointing.

Cheers
Salil.