linux-kernel - Re: [PATCH v5 03/22] cc_platform: Add new attribute to prevent ACPI memory hotplug

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Message-ID: <CAJZ5v0jEJNdmkidvcOiRn+OVt01D5095t+nyXaJHKsqEAOvcBQ@mail.gmail.com>
Date:   Wed, 22 Jun 2022 13:45:01 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Kai Huang <kai.huang@...el.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kvm-devel <kvm@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Len Brown <len.brown@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andi Kleen <ak@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        isaku.yamahata@...el.com, Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH v5 03/22] cc_platform: Add new attribute to prevent ACPI
 memory hotplug

On Wed, Jun 22, 2022 at 1:16 PM Kai Huang <kai.huang@...el.com> wrote:
>
> Platforms with confidential computing technology may not support ACPI
> memory 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 memory hotplug event, it is likely a
> BIOS bug.  For ACPI memory hot-add, the kernel should speak out this is
> a BIOS bug and reject the new memory.  For hot-removal, for simplicity
> just assume the kernel cannot continue to work normally, and just BUG().
>
> Add a new attribute CC_ATTR_ACPI_MEMORY_HOTPLUG_DISABLED to indicate the
> platform doesn't support ACPI memory hotplug, so that kernel can handle
> ACPI memory hotplug events for such platform.
>
> In acpi_memory_device_{add|remove}(), add early check against this
> attribute and handle accordingly if it is set.
>
> Signed-off-by: Kai Huang <kai.huang@...el.com>
> ---
>  drivers/acpi/acpi_memhotplug.c | 23 +++++++++++++++++++++++
>  include/linux/cc_platform.h    | 10 ++++++++++
>  2 files changed, 33 insertions(+)
>
> diff --git a/drivers/acpi/acpi_memhotplug.c b/drivers/acpi/acpi_memhotplug.c
> index 24f662d8bd39..94d6354ea453 100644
> --- a/drivers/acpi/acpi_memhotplug.c
> +++ b/drivers/acpi/acpi_memhotplug.c
> @@ -15,6 +15,7 @@
>  #include <linux/acpi.h>
>  #include <linux/memory.h>
>  #include <linux/memory_hotplug.h>
> +#include <linux/cc_platform.h>
>
>  #include "internal.h"
>
> @@ -291,6 +292,17 @@ static int acpi_memory_device_add(struct acpi_device *device,
>         if (!device)
>                 return -EINVAL;
>
> +       /*
> +        * 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 memory device.
> +        */
> +       if (cc_platform_has(CC_ATTR_ACPI_MEMORY_HOTPLUG_DISABLED)) {

Same comment as for the acpi_processor driver: this will affect the
initialization too and it would be cleaner to reset the
.hotplug.enabled flag of the scan handler.

> +               dev_err(&device->dev, "[BIOS bug]: Platform doesn't support ACPI memory hotplug. New memory device ignored.\n");
> +               return -EINVAL;
> +       }
> +
>         mem_device = kzalloc(sizeof(struct acpi_memory_device), GFP_KERNEL);
>         if (!mem_device)
>                 return -ENOMEM;
> @@ -334,6 +346,17 @@ static void acpi_memory_device_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 is 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 memory hotplug. BUG().\n");
> +               BUG();
> +       }
> +
>         mem_device = acpi_driver_data(device);
>         acpi_memory_remove_memory(mem_device);
>         acpi_memory_device_free(mem_device);
> diff --git a/include/linux/cc_platform.h b/include/linux/cc_platform.h
> index 9ce9256facc8..b831c24bd7f6 100644
> --- a/include/linux/cc_platform.h
> +++ b/include/linux/cc_platform.h
> @@ -93,6 +93,16 @@ enum cc_attr {
>          * Examples include TDX platform.
>          */
>         CC_ATTR_ACPI_CPU_HOTPLUG_DISABLED,
> +
> +       /**
> +        * @CC_ATTR_ACPI_MEMORY_HOTPLUG_DISABLED: ACPI memory hotplug is
> +        *                                        not supported.
> +        *
> +        * The platform/os does not support ACPI memory hotplug.
> +        *
> +        * Examples include TDX platform.
> +        */
> +       CC_ATTR_ACPI_MEMORY_HOTPLUG_DISABLED,
>  };
>
>  #ifdef CONFIG_ARCH_HAS_CC_PLATFORM
> --
> 2.36.1
>