Re: [edk2-devel] [PATCH v2] OvmfPkg/PlatformInitLib: catch QEMU's CPU hotplug reg block regression

["Ard Biesheuvel" <ardb@kernel.org>]

On Fri, 13 Jan 2023 at 13:22, Gerd Hoffmann <kraxel@redhat.com> wrote:
>
> On Fri, Jan 13, 2023 at 11:10:54AM +0100, Laszlo Ersek wrote:
> > On 1/13/23 10:32, Gerd Hoffmann wrote:
> > > On Fri, Jan 13, 2023 at 07:03:54AM +0100, Gerd Hoffmann wrote:
> > >>   Hi,
> > >>
> > >>> - QEMU can be configured with other compat properties on the command
> > >>> line so that "CPU hotplug with SMI" and "CPU hot-unplug with SMI" *not*
> > >>> be offered to the firmware. Then QEMU will reject hotplug attempts, and
> > >>> the SMM hotplug code in edk2 will not be triggered by the (virtual)
> > >>> hardware.
> > >>
> > >> Can we have edk2 print instructions for that in the error message?
> > >
> > > This seems to be:
> > >
> > >     qemu -M q35 \
> > >         -global ICH9-LPC.x-smi-cpu-hotplug=off \
> > >         -global ICH9-LPC.x-smi-cpu-hotunplug=off
> >
> > Yes, those are the flags.
> >
> > > But it appears to not work.
> >
> > They should work, but they take effect in QEMU, and not in the firmware.
> > These knobs control what CPU hot(un)plug+SMI features QEMU exposes to
> > the guest fw, via fw_cfg,
>
> Ok, I see, only the SMM code actually checks that.
>
> > In particular the firmware makes no further decisions based on whether
> > QEMU advertized some of these features.
>
> I was thinking the other way around:  When cpu hotplug is disabled in
> qemu it should be safe to skip the whole cpu hotplug checking dance.
> See test patch below.
>
> That would give us a config switch (turn off cpu hotplug support) which
> would allow edk2 run on qemu versions with broken cpu hotplug.
>
> Does the idea look sane or do I miss something?
>

I cannot review the actual patch due to lack of x86/smm/qemu/hotplug
knowledge, but if this allows us to merge Laszlo's fix without
breaking all current QEMU/x86 TCG users, I'm all for it.


>
> commit bd2e36eba35268ab46c0125d2b9125391ea6f9fc
> Author: Gerd Hoffmann <kraxel@redhat.com>
> Date:   Fri Jan 13 13:07:36 2023 +0100
>
>     skip cpu present checking when hotplug is off
>
> diff --git a/OvmfPkg/Library/PlatformInitLib/Platform.c b/OvmfPkg/Library/PlatformInitLib/Platform.c
> index 13348afb4890..2b0f0c836f85 100644
> --- a/OvmfPkg/Library/PlatformInitLib/Platform.c
> +++ b/OvmfPkg/Library/PlatformInitLib/Platform.c
> @@ -415,8 +415,9 @@ PlatformMaxCpuCountInitialization (
>    IN OUT EFI_HOB_PLATFORM_INFO  *PlatformInfoHob
>    )
>  {
> -  UINT16  BootCpuCount = 0;
> -  UINT32  MaxCpuCount;
> +  UINT16   BootCpuCount = 0;
> +  UINT32   MaxCpuCount;
> +  BOOLEAN  CpuHotplugSupported = FALSE;
>
>    //
>    // Try to fetch the boot CPU count.
> @@ -424,6 +425,31 @@ PlatformMaxCpuCountInitialization (
>    if (QemuFwCfgIsAvailable ()) {
>      QemuFwCfgSelectItem (QemuFwCfgItemSmpCpuCount);
>      BootCpuCount = QemuFwCfgRead16 ();
> +    DEBUG ((DEBUG_INFO, "%a: BootCpuCount: %d\n", __FUNCTION__, BootCpuCount));
> +  }
> +
> +  {
> +    FIRMWARE_CONFIG_ITEM  SupportedFeaturesItem;
> +    UINTN                 SupportedFeaturesSize;
> +    UINT64                mSmiFeatures;
> +    EFI_STATUS            Status;
> +
> +    Status = QemuFwCfgFindFile (
> +               "etc/smi/supported-features",
> +               &SupportedFeaturesItem,
> +               &SupportedFeaturesSize
> +               );
> +
> +    if (EFI_ERROR (Status)) {
> +      DEBUG ((DEBUG_INFO, "%a: etc/smi/supported-features: %r\n", __FUNCTION__, Status));
> +    } else {
> +      QemuFwCfgSelectItem (SupportedFeaturesItem);
> +      QemuFwCfgReadBytes (sizeof mSmiFeatures, &mSmiFeatures);
> +      DEBUG ((DEBUG_INFO, "%a: etc/smi/supported-features: 0x%x\n", __FUNCTION__, mSmiFeatures));
> +      if (mSmiFeatures & (BIT1 /* hotplug */ | BIT2 /* hotunplug */)) {
> +        CpuHotplugSupported = TRUE;
> +      }
> +    }
>    }
>
>    if (BootCpuCount == 0) {
> @@ -435,6 +461,9 @@ PlatformMaxCpuCountInitialization (
>      //
>      DEBUG ((DEBUG_WARN, "%a: boot CPU count unavailable\n", __FUNCTION__));
>      MaxCpuCount = PlatformInfoHob->DefaultMaxCpuNumber;
> +  } else if (!CpuHotplugSupported) {
> +    DEBUG ((DEBUG_INFO, "%a: CPU hotplug support not available\n", __FUNCTION__));
> +    MaxCpuCount = BootCpuCount;
>    } else {
>      //
>      // We will expose BootCpuCount to MpInitLib. MpInitLib will count APs up to
>