Re: [RFC PATCH 2/4] mm: Add support for unaccepted memory hotplug

["David Hildenbrand (Red Hat)" <david@kernel.org>]

On 11/25/25 18:57, Pratik R. Sampat wrote:
> The unaccepted memory structure currently only supports accepting memory
> present at boot time. The unaccepted table uses a fixed-size bitmap
> reserved in memblock based on the initial memory layout, preventing
> dynamic addition of memory ranges after boot. This causes guest
> termination when memory is hot-added in a secure virtual machine due to
> accessing pages that have not transitioned to private before use.
> 
> Extend the unaccepted memory framework to handle hotplugged memory by
> dynamically managing the unaccepted bitmap. Allocate a new bitmap when
> hotplugged ranges exceed the reserved bitmap capacity and switch to
> kernel-managed allocation.
> 
> Hotplugged memory also follows the same acceptance policy using the
> accept_memory=[eager|lazy] kernel parameter to accept memory either
> up-front when added or before first use.
> 
> Signed-off-by: Pratik R. Sampat <prsampat@amd.com>
> ---
>   arch/x86/boot/compressed/efi.h                |  1 +
>   .../firmware/efi/libstub/unaccepted_memory.c  |  1 +
>   drivers/firmware/efi/unaccepted_memory.c      | 83 +++++++++++++++++++
>   include/linux/efi.h                           |  1 +
>   include/linux/mm.h                            | 11 +++
>   mm/memory_hotplug.c                           |  7 ++
>   mm/page_alloc.c                               |  2 +
>   7 files changed, 106 insertions(+)
> 
> diff --git a/arch/x86/boot/compressed/efi.h b/arch/x86/boot/compressed/efi.h
> index 4f7027f33def..a220a1966cae 100644
> --- a/arch/x86/boot/compressed/efi.h
> +++ b/arch/x86/boot/compressed/efi.h
> @@ -102,6 +102,7 @@ struct efi_unaccepted_memory {
>   	u32 unit_size;
>   	u64 phys_base;
>   	u64 size;
> +	bool mem_reserved;
>   	unsigned long *bitmap;
>   };
>   
> diff --git a/drivers/firmware/efi/libstub/unaccepted_memory.c b/drivers/firmware/efi/libstub/unaccepted_memory.c
> index c1370fc14555..b16bd61c12bf 100644
> --- a/drivers/firmware/efi/libstub/unaccepted_memory.c
> +++ b/drivers/firmware/efi/libstub/unaccepted_memory.c
> @@ -83,6 +83,7 @@ efi_status_t allocate_unaccepted_bitmap(__u32 nr_desc,
>   	unaccepted_table->unit_size = EFI_UNACCEPTED_UNIT_SIZE;
>   	unaccepted_table->phys_base = unaccepted_start;
>   	unaccepted_table->size = bitmap_size;
> +	unaccepted_table->mem_reserved = true;
>   	memset(unaccepted_table->bitmap, 0, bitmap_size);
>   
>   	status = efi_bs_call(install_configuration_table,
> diff --git a/drivers/firmware/efi/unaccepted_memory.c b/drivers/firmware/efi/unaccepted_memory.c
> index 4479aad258f8..8537812346e2 100644
> --- a/drivers/firmware/efi/unaccepted_memory.c
> +++ b/drivers/firmware/efi/unaccepted_memory.c
> @@ -218,6 +218,89 @@ bool range_contains_unaccepted_memory(phys_addr_t start, unsigned long size)
>   	return ret;
>   }
>   
> +static int extend_unaccepted_bitmap(phys_addr_t mem_range_start,
> +				    unsigned long mem_range_size)
> +{
> +	struct efi_unaccepted_memory *unacc_tbl;
> +	unsigned long *old_bitmap, *new_bitmap;
> +	phys_addr_t start, end, mem_range_end;
> +	u64 phys_base, size, unit_size;
> +	unsigned long flags;
> +
> +	unacc_tbl = efi_get_unaccepted_table();
> +	if (!unacc_tbl || !unacc_tbl->unit_size)
> +		return -EIO;
> +
> +	unit_size = unacc_tbl->unit_size;
> +	phys_base = unacc_tbl->phys_base;
> +
> +	mem_range_end = round_up(mem_range_start + mem_range_size, unit_size);
> +	size = DIV_ROUND_UP(mem_range_end - phys_base, unit_size * BITS_PER_BYTE);
> +
> +	/* Translate to offsets from the beginning of the bitmap */
> +	start = mem_range_start - phys_base;
> +	end = mem_range_end - phys_base;
> +
> +	old_bitmap = efi_get_unaccepted_bitmap();
> +	if (!old_bitmap)
> +		return -EIO;
> +
> +	/* If the bitmap is already large enough, just set the bits */
> +	if (unacc_tbl->size >= size) {
> +		spin_lock_irqsave(&unaccepted_memory_lock, flags);
> +		bitmap_set(old_bitmap, start / unit_size, (end - start) / unit_size);
> +		spin_unlock_irqrestore(&unaccepted_memory_lock, flags);
> +
> +		return 0;
> +	}
> +
> +	/* Reserved memblocks cannot be extended so allocate a new bitmap */
> +	if (unacc_tbl->mem_reserved) {
> +		new_bitmap = kzalloc(size, GFP_KERNEL);
> +		if (!new_bitmap)
> +			return -ENOMEM;
> +
> +		spin_lock_irqsave(&unaccepted_memory_lock, flags);
> +		memcpy(new_bitmap, old_bitmap, unacc_tbl->size);
> +		unacc_tbl->mem_reserved = false;
> +		free_reserved_area(old_bitmap, old_bitmap + unacc_tbl->size, -1, NULL);
> +		spin_unlock_irqrestore(&unaccepted_memory_lock, flags);
> +	} else {
> +		new_bitmap = krealloc(old_bitmap, size, GFP_KERNEL);
> +		if (!new_bitmap)
> +			return -ENOMEM;
> +
> +		/* Zero the bitmap from the range it was extended from */
> +		memset(new_bitmap + unacc_tbl->size, 0, size - unacc_tbl->size);
> +	}
> +
> +	bitmap_set(new_bitmap, start / unit_size, (end - start) / unit_size);
> +
> +	spin_lock_irqsave(&unaccepted_memory_lock, flags);
> +	unacc_tbl->size = size;
> +	unacc_tbl->bitmap = (unsigned long *)__pa(new_bitmap);
> +	spin_unlock_irqrestore(&unaccepted_memory_lock, flags);
> +
> +	return 0;
> +}
> +
> +int accept_hotplug_memory(phys_addr_t mem_range_start, unsigned long mem_range_size)
> +{
> +	int ret;
> +
> +	if (!IS_ENABLED(CONFIG_UNACCEPTED_MEMORY))
> +		return 0;
> +
> +	ret = extend_unaccepted_bitmap(mem_range_start, mem_range_size);
> +	if (ret)
> +		return ret;
> +
> +	if (!mm_lazy_accept_enabled())
> +		accept_memory(mem_range_start, mem_range_size);
> +
> +	return 0;
> +}
> +
>   #ifdef CONFIG_PROC_VMCORE
>   static bool unaccepted_memory_vmcore_pfn_is_ram(struct vmcore_cb *cb,
>   						unsigned long pfn)
> diff --git a/include/linux/efi.h b/include/linux/efi.h
> index a74b393c54d8..1021eb78388f 100644
> --- a/include/linux/efi.h
> +++ b/include/linux/efi.h
> @@ -545,6 +545,7 @@ struct efi_unaccepted_memory {
>   	u32 unit_size;
>   	u64 phys_base;
>   	u64 size;
> +	bool mem_reserved;
>   	unsigned long *bitmap;
>   };
>   
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 1ae97a0b8ec7..bb43876e6c47 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4077,6 +4077,9 @@ int set_anon_vma_name(unsigned long addr, unsigned long size,
>   
>   bool range_contains_unaccepted_memory(phys_addr_t start, unsigned long size);
>   void accept_memory(phys_addr_t start, unsigned long size);
> +int accept_hotplug_memory(phys_addr_t mem_range_start,
> +			  unsigned long mem_range_size);
> +bool mm_lazy_accept_enabled(void);
>   
>   #else
>   
> @@ -4090,6 +4093,14 @@ static inline void accept_memory(phys_addr_t start, unsigned long size)
>   {
>   }
>   
> +static inline int accept_hotplug_memory(phys_addr_t mem_range_start,
> +					unsigned long mem_range_size)
> +{
> +	return 0;
> +}
> +
> +static inline bool mm_lazy_accept_enabled(void) { return false; }
> +
>   #endif
>   
>   static inline bool pfn_is_unaccepted_memory(unsigned long pfn)
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 74318c787715..bf8086682b66 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1581,6 +1581,13 @@ int add_memory_resource(int nid, struct resource *res, mhp_t mhp_flags)
>   	if (!strcmp(res->name, "System RAM"))
>   		firmware_map_add_hotplug(start, start + size, "System RAM");
>   
> +	ret = accept_hotplug_memory(start, size);

What makes this special that we have to have "hotplug_memory" as part of 
the name?

Staring at the helper itself, there isn't anything really hotplug 
specific happening in there except extending the bitmap, maybe?

-- 
Cheers

David