Re: [PATCH v5 02/22] cc_platform: Add new attribute to prevent ACPI CPU hotplug

[Kai Huang]

On Mon, 2022-06-27 at 10:01 +0200, Igor Mammedov wrote:
> On Thu, 23 Jun 2022 12:01:48 +1200
> Kai Huang <kai.huang@xxxxxxxxx> wrote:
> 
> > On Wed, 2022-06-22 at 13:42 +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
> > > > 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@xxxxxxxxx>
> > > > ---
> > > >  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.  
> > 
> > Hi Rafael,
> > 
> > Thanks for the review.  By "affect initialization" did you mean this
> > acpi_processor_add() is also called during kernel boot when any logical cpu is
> > brought up?  Or do you mean ACPI CPU hotplug can also happen during kernel boot
> > (after acpi_processor_init())?
> > 
> > I see acpi_processor_init() calls acpi_processor_check_duplicates() which calls
> > acpi_evaluate_object() but I don't know details of ACPI so I don't know whether
> > this would trigger acpi_processor_add().
> > 
> > One thing is TDX doesn't support ACPI CPU hotplug is an architectural thing, so
> > it is illegal even if it happens during kernel boot.  Dave's idea is the kernel
> > should  speak out loudly if physical CPU hotplug indeed happened on (BIOS) TDX-
> > enabled platforms.  Otherwise perhaps we can just give up initializing the ACPI
> > CPU hotplug in acpi_processor_init(), something like below?
> 
> The thing is that by the time ACPI machinery kicks in, physical hotplug
> has already happened and in case of (kvm+qemu+ovmf hypervisor combo)
> firmware has already handled it somehow and handed it over to ACPI.
> If you say it's architectural thing then cpu hotplug is platform/firmware
> bug and should be disabled there instead of working around it in the kernel.
> 
> Perhaps instead of 'preventing' hotplug, complain/panic and be done with it.

Hi Igor,

Thanks for feedback.  Yes the current implementation actually reports CPU hot-
add as BIOS bug.  I think I can report BIOS bug for hot-removal too.  And
currently I actually used BUG() for the hot-removal case.  For hot-add I didn't
use BUG() but rejected the new CPU as the latter is more conservative. 

Hi Rafael,

I am not sure I got what you mean by "This will affect initialization, not just
hotplug AFAICS", could you elaborate a little bit?  Thanks.


-- 
Thanks,
-Kai