linux-kernel - Re: [PATCH v2 1/2] mm/memory_hotplug: Add support to accept memory during hot-add

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Message-Id: <20260112144314.69b850afad9ee67143f30a85@linux-foundation.org>
Date: Mon, 12 Jan 2026 14:43:14 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: "Pratik R. Sampat" <prsampat@....com>
Cc: linux-mm@...ck.org, linux-coco@...ts.linux.dev, x86@...nel.org,
 linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
 bp@...en8.de, dave.hansen@...ux.intel.com, kas@...nel.org, ardb@...nel.org,
 david@...nel.org, osalvador@...e.de, thomas.lendacky@....com,
 michael.roth@....com
Subject: Re: [PATCH v2 1/2] mm/memory_hotplug: Add support to accept memory
 during hot-add

On Mon, 12 Jan 2026 16:23:37 -0600 "Pratik R. Sampat" <prsampat@....com> wrote:

> 
> 
> On 1/12/26 3:04 PM, Andrew Morton wrote:
> > On Mon, 12 Jan 2026 14:22:59 -0600 "Pratik R. Sampat" <prsampat@....com> wrote:
> > 
> >> Confidential computing guests require memory to be accepted before use.
> >> The unaccepted memory bitmap maintained by firmware does not track
> >> hotplugged memory ranges.
> >>
> >> Call arch_accept_memory() during the hot-add path to explicitly validate
> >> and transition the newly added memory to a private state, making it
> >> usable by the guest.
> >>
> >> Signed-off-by: Pratik R. Sampat <prsampat@....com>
> >> ---
> >>  mm/memory_hotplug.c | 4 ++++
> >>  1 file changed, 4 insertions(+)
> >>
> >> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> >> index a63ec679d861..8cfbf0541430 100644
> >> --- a/mm/memory_hotplug.c
> >> +++ b/mm/memory_hotplug.c
> >> @@ -38,6 +38,7 @@
> >>  #include <linux/node.h>
> >>  
> >>  #include <asm/tlbflush.h>
> >> +#include <asm/unaccepted_memory.h>
> > 
> > This only exists for x86!
> 
> Ah, I missed that entirely. Thanks for catching that.
> 
> Probably not the best option to have a generic unaccepted_memory.h as well.
> Maybe, I should have arch_[un]accept_memory() definitions within mm.h wrapped
> within CONFIG_UNACCEPTED_MEMORY instead so that its cleaner.

Something like that.

The idiomatic Linus way is to use

#ifndef arch_accept_memory
#define arch_accept_memory ...
#endif

Lots of prior art here:

	grep -r include/linux "ifndef arch_"

Oh, arch_get_idle_state_flags() got it all wrong.

	#ifdef CONFIG_ACPI_PROCESSOR_IDLE
	#ifndef arch_get_idle_state_flags
	static inline unsigned int arch_get_idle_state_flags(u32 arch_flags)
	{
		return 0;
	}
	#endif
	#endif /* CONFIG_ACPI_PROCESSOR_IDLE */

- shouldn't have needed "ifdef CONFIG_ACPI_PROCESSOR_IDLE"

- should have appended

	#define arch_get_idle_state_flags arch_get_idle_state_flags

  in case cpp hit the same lines a second time.