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    SubjectRe: [PATCH v5 02/22] cc_platform: Add new attribute to prevent ACPI CPU hotplug
    On Wed, Jun 22, 2022 at 1:16 PM Kai Huang <kai.huang@intel.com> wrote:
    >
    > Platforms with confidential computing technology may not support ACPI
    > CPU hotplug when such technology is enabled by the BIOS. Examples
    > include Intel platforms which support Intel Trust Domain Extensions
    > (TDX).
    >
    > If the kernel ever receives ACPI CPU hotplug event, it is likely a BIOS
    > bug. For ACPI CPU hot-add, the kernel should speak out this is a BIOS
    > bug and reject the new CPU. For hot-removal, for simplicity just assume
    > the kernel cannot continue to work normally, and BUG().
    >
    > Add a new attribute CC_ATTR_ACPI_CPU_HOTPLUG_DISABLED to indicate the
    > platform doesn't support ACPI CPU hotplug, so that kernel can handle
    > ACPI CPU hotplug events for such platform. The existing attribute
    > CC_ATTR_HOTPLUG_DISABLED is for software CPU hotplug thus doesn't fit.
    >
    > In acpi_processor_{add|remove}(), add early check against this attribute
    > and handle accordingly if it is set.
    >
    > Also take this chance to rename existing CC_ATTR_HOTPLUG_DISABLED to
    > CC_ATTR_CPU_HOTPLUG_DISABLED as it is for software CPU hotplug.
    >
    > Signed-off-by: Kai Huang <kai.huang@intel.com>
    > ---
    > arch/x86/coco/core.c | 2 +-
    > drivers/acpi/acpi_processor.c | 23 +++++++++++++++++++++++
    > include/linux/cc_platform.h | 15 +++++++++++++--
    > kernel/cpu.c | 2 +-
    > 4 files changed, 38 insertions(+), 4 deletions(-)
    >
    > diff --git a/arch/x86/coco/core.c b/arch/x86/coco/core.c
    > index 4320fadae716..1bde1af75296 100644
    > --- a/arch/x86/coco/core.c
    > +++ b/arch/x86/coco/core.c
    > @@ -20,7 +20,7 @@ static bool intel_cc_platform_has(enum cc_attr attr)
    > {
    > switch (attr) {
    > case CC_ATTR_GUEST_UNROLL_STRING_IO:
    > - case CC_ATTR_HOTPLUG_DISABLED:
    > + case CC_ATTR_CPU_HOTPLUG_DISABLED:
    > case CC_ATTR_GUEST_MEM_ENCRYPT:
    > case CC_ATTR_MEM_ENCRYPT:
    > return true;
    > diff --git a/drivers/acpi/acpi_processor.c b/drivers/acpi/acpi_processor.c
    > index 6737b1cbf6d6..b960db864cd4 100644
    > --- a/drivers/acpi/acpi_processor.c
    > +++ b/drivers/acpi/acpi_processor.c
    > @@ -15,6 +15,7 @@
    > #include <linux/kernel.h>
    > #include <linux/module.h>
    > #include <linux/pci.h>
    > +#include <linux/cc_platform.h>
    >
    > #include <acpi/processor.h>
    >
    > @@ -357,6 +358,17 @@ static int acpi_processor_add(struct acpi_device *device,
    > struct device *dev;
    > int result = 0;
    >
    > + /*
    > + * If the confidential computing platform doesn't support ACPI
    > + * memory hotplug, the BIOS should never deliver such event to
    > + * the kernel. Report ACPI CPU hot-add as a BIOS bug and ignore
    > + * the new CPU.
    > + */
    > + if (cc_platform_has(CC_ATTR_ACPI_CPU_HOTPLUG_DISABLED)) {

    This will affect initialization, not just hotplug AFAICS.

    You should reset the .hotplug.enabled flag in processor_handler to
    false instead.

    > + dev_err(&device->dev, "[BIOS bug]: Platform doesn't support ACPI CPU hotplug. New CPU ignored.\n");
    > + return -EINVAL;
    > + }
    > +
    > pr = kzalloc(sizeof(struct acpi_processor), GFP_KERNEL);
    > if (!pr)
    > return -ENOMEM;
    > @@ -434,6 +446,17 @@ static void acpi_processor_remove(struct acpi_device *device)
    > if (!device || !acpi_driver_data(device))
    > return;
    >
    > + /*
    > + * The confidential computing platform is broken if ACPI memory
    > + * hot-removal isn't supported but it happened anyway. Assume
    > + * it's not guaranteed that the kernel can continue to work
    > + * normally. Just BUG().
    > + */
    > + if (cc_platform_has(CC_ATTR_ACPI_CPU_HOTPLUG_DISABLED)) {
    > + dev_err(&device->dev, "Platform doesn't support ACPI CPU hotplug. BUG().\n");
    > + BUG();
    > + }
    > +
    > pr = acpi_driver_data(device);
    > if (pr->id >= nr_cpu_ids)
    > goto out;
    > diff --git a/include/linux/cc_platform.h b/include/linux/cc_platform.h
    > index 691494bbaf5a..9ce9256facc8 100644
    > --- a/include/linux/cc_platform.h
    > +++ b/include/linux/cc_platform.h
    > @@ -74,14 +74,25 @@ enum cc_attr {
    > CC_ATTR_GUEST_UNROLL_STRING_IO,
    >
    > /**
    > - * @CC_ATTR_HOTPLUG_DISABLED: Hotplug is not supported or disabled.
    > + * @CC_ATTR_CPU_HOTPLUG_DISABLED: CPU hotplug is not supported or
    > + * disabled.
    > *
    > * The platform/OS is running as a guest/virtual machine does not
    > * support CPU hotplug feature.
    > *
    > * Examples include TDX Guest.
    > */
    > - CC_ATTR_HOTPLUG_DISABLED,
    > + CC_ATTR_CPU_HOTPLUG_DISABLED,
    > +
    > + /**
    > + * @CC_ATTR_ACPI_CPU_HOTPLUG_DISABLED: ACPI CPU hotplug is not
    > + * supported.
    > + *
    > + * The platform/OS does not support ACPI CPU hotplug.
    > + *
    > + * Examples include TDX platform.
    > + */
    > + CC_ATTR_ACPI_CPU_HOTPLUG_DISABLED,
    > };
    >
    > #ifdef CONFIG_ARCH_HAS_CC_PLATFORM
    > diff --git a/kernel/cpu.c b/kernel/cpu.c
    > index edb8c199f6a3..966772cce063 100644
    > --- a/kernel/cpu.c
    > +++ b/kernel/cpu.c
    > @@ -1191,7 +1191,7 @@ static int cpu_down_maps_locked(unsigned int cpu, enum cpuhp_state target)
    > * If the platform does not support hotplug, report it explicitly to
    > * differentiate it from a transient offlining failure.
    > */
    > - if (cc_platform_has(CC_ATTR_HOTPLUG_DISABLED))
    > + if (cc_platform_has(CC_ATTR_CPU_HOTPLUG_DISABLED))
    > return -EOPNOTSUPP;
    > if (cpu_hotplug_disabled)
    > return -EBUSY;
    > --
    > 2.36.1
    >

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