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

[Rafael J. Wysocki]

On Thu, Jun 23, 2022 at 2:09 AM Kai Huang <kai.huang@xxxxxxxxx> wrote:
>
> On Wed, 2022-06-22 at 13:45 +0200, Rafael J. Wysocki wrote:
> > On Wed, Jun 22, 2022 at 1:16 PM Kai Huang <kai.huang@xxxxxxxxx> 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@xxxxxxxxx>
> > > ---
> > >  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.
> >
> >
>
> Hi Rafael,
>
> Thanks for review.  The same to the ACPI CPU hotplug handling, this is illegal
> also during kernel boot.

What do you mean?

Is it not correct to enumerate any memory device through ACPI at all?

>  If we just want to disable, then perhaps something like below?
>
> --- a/drivers/acpi/acpi_memhotplug.c
> +++ b/drivers/acpi/acpi_memhotplug.c
> @@ -366,6 +366,9 @@ static bool __initdata acpi_no_memhotplug;
>
>  void __init acpi_memory_hotplug_init(void)
>  {
> +       if (cc_platform_has(CC_ATTR_ACPI_MEMORY_HOTPLUG_DISABLED))
> +               acpi_no_memhotplug = true;
> +

This looks fine to me if the above is the case, but you need to modify
the changelog to match.

>         if (acpi_no_memhotplug) {
>                 memory_device_handler.attach = NULL;
>                 acpi_scan_add_handler(&memory_device_handler);
>
>
> --