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

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Date:   Wed, 29 Jun 2022 11:49:14 +1200
From:   Kai Huang <kai.huang@...el.com>
To:     Igor Mammedov <imammedo@...hat.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>
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 Tue, 2022-06-28 at 14:01 +0200, Igor Mammedov wrote:
> On Wed, 22 Jun 2022 13:45:01 +0200
> "Rafael J. Wysocki" <rafael@...nel.org> wrote:
> 
> > 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(c)) {  
> > 
> > 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.
> 
> with QEMU, it is likely broken when memory is added as
>   '-device pc-dimm'
> on CLI since it's advertised only as device node in DSDT.
> 
> 

Hi Rafael,  Igor,

On my test machine, the acpi_memory_device_add() is not called for system
memory.  It probably because my machine doesn't have memory device in ACPI.

I don't know whether we can have any memory device in ACPI if such memory is
present during boot?  Any comments here?

And CC_ATTR_ACPI_MEMORY_HOTPLUG_DISABLED is only true on TDX bare-metal system,
but cannot be true in Qemu guest.  But yes if this flag ever becomes true in
guest, then I think we may have problem here.  I will do more study around ACPI.
Thanks for comments!

-- 
Thanks,
-Kai