[PATCH v2 1/6] efi/libstub: add a helper for the top of usable RAM

[Kiryl Shutsemau]

On Fri, Aug 21, 2026 at 03:06:01AM -0700, Breno Leitao wrote:
> diff --git a/drivers/firmware/efi/libstub/mem.c b/drivers/firmware/efi/libstub/mem.c
> index 59f3f83de50c2..777f1f180c66e 100644
> --- a/drivers/firmware/efi/libstub/mem.c
> +++ b/drivers/firmware/efi/libstub/mem.c
> @@ -64,6 +64,59 @@ efi_status_t efi_get_memory_map(struct efi_boot_memmap **map,
>  	return EFI_SUCCESS;
>  }
>  
> +/**
> + * efi_get_ram_top() - find the top of usable RAM
> + * @top:	on return, the end of the highest memory map entry that becomes
> + *		usable RAM
> + *
> + * Walk the UEFI memory map for the entry types that become usable RAM, the set
> + * setup_e820() maps to E820_TYPE_RAM, and return the highest address any of
> + * them reaches. Memory a confidential guest has not accepted yet counts as
> + * well, since it becomes RAM once accepted. This is what the stub has in place
> + * of max_pfn, which is only set once the kernel proper is up.
> + *
> + * Memory the firmware hot-adds later is not described by the memory map and so
> + * is not accounted for here.
> + *
> + * Return:	status code
> + */
> +efi_status_t efi_get_ram_top(u64 *top)
> +{
> +	struct efi_boot_memmap *map __free(efi_pool) = NULL;
> +	efi_status_t status;
> +	u64 ram_top = 0;
> +	int i, nr_desc;
> +
> +	status = efi_get_memory_map(&map, false);
> +	if (status != EFI_SUCCESS)
> +		return status;
> +
> +	nr_desc = map->map_size / map->desc_size;
> +	for (i = 0; i < nr_desc; i++) {
> +		efi_memory_desc_t *d;
> +
> +		d = efi_memdesc_ptr((unsigned long)map->map, map->desc_size, i);
> +		switch (d->type) {
> +		case EFI_LOADER_CODE:
> +		case EFI_LOADER_DATA:
> +		case EFI_BOOT_SERVICES_CODE:
> +		case EFI_BOOT_SERVICES_DATA:
> +		case EFI_CONVENTIONAL_MEMORY:
> +		case EFI_UNACCEPTED_MEMORY:
> +			ram_top = max(ram_top,
> +				      d->phys_addr + d->num_pages * EFI_PAGE_SIZE);
> +			break;
> +		default:
> +			break;
> +		}
> +	}

You model it after setup_e820() which is x86-specific.

On ARM is_memory() adds anything with WB/WT/WC, and is_usable_memory()
also counts EFI_ACPI_RECLAIM_MEMORY and EFI_PERSISTENT_MEMORY.


-- 
  Kiryl Shutsemau / Kirill A. Shutemov