[PATCH v3 10/11] arm64: allow kernel Image to be loaded anywhere in physical memory

[mark.rutland@arm.com (Mark Rutland)]

On Fri, Apr 10, 2015 at 02:53:54PM +0100, Ard Biesheuvel wrote:
> This relaxes the kernel Image placement requirements, so that it
> may be placed at any 2 MB aligned offset in physical memory.
> 
> This is accomplished by ignoring PHYS_OFFSET when installing
> memblocks, and accounting for the apparent virtual offset of
> the kernel Image (in addition to the 64 MB that is is moved
> below PAGE_OFFSET). As a result, virtual address references
> below PAGE_OFFSET are correctly mapped onto physical references
> into the kernel Image regardless of where it sits in memory.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---
>  Documentation/arm64/booting.txt | 17 +++++++----------
>  arch/arm64/mm/init.c            | 32 +++++++++++++++++++-------------
>  2 files changed, 26 insertions(+), 23 deletions(-)
> 
> diff --git a/Documentation/arm64/booting.txt b/Documentation/arm64/booting.txt
> index 6396460f6085..811d93548bdc 100644
> --- a/Documentation/arm64/booting.txt
> +++ b/Documentation/arm64/booting.txt
> @@ -110,16 +110,13 @@ Header notes:
>    depending on selected features, and is effectively unbound.
>  
>  The Image must be placed text_offset bytes from a 2MB aligned base
> -address near the start of usable system RAM and called there. Memory
> -below that base address is currently unusable by Linux, and therefore it
> -is strongly recommended that this location is the start of system RAM.
> -At least image_size bytes from the start of the image must be free for
> -use by the kernel.
> -
> -Any memory described to the kernel (even that below the 2MB aligned base
> -address) which is not marked as reserved from the kernel e.g. with a
> -memreserve region in the device tree) will be considered as available to
> -the kernel.
> +address anywhere in usable system RAM and called there. At least
> +image_size bytes from the start of the image must be free for use
> +by the kernel.
> +
> +Any memory described to the kernel which is not marked as reserved from
> +the kernel e.g. with a memreserve region in the device tree) will be
> +considered as available to the kernel.

As with the other docs changes we'll need a note w.r.t. the behaviour
older kernels. This might also be worth a feature bitmap bit so loaders
can do the best thing for the given kernel version.

> +	/*
> +	 * Set memstart_addr to the base of the lowest physical memory region,
> +	 * rounded down to PUD/PMD alignment so we can map it efficiently.
> +	 * Since this also affects the apparent offset of the kernel image in
> +	 * the virtual address space, increase image_offset by the same amount
> +	 * that we decrease memstart_addr.
> +	 */
> +	if (!memstart_addr || memstart_addr > base) {
> +		u64 new_memstart_addr;
> +
> +		if (IS_ENABLED(CONFIG_ARM64_64K_PAGES))
> +			new_memstart_addr = base & PMD_MASK;
> +		else
> +			new_memstart_addr = base & PUD_MASK;
> +
> +		image_offset += memstart_addr - new_memstart_addr;
> +		memstart_addr = new_memstart_addr;
> +	}

There's one slight snag with this. Given sufficient memory (e.g. more
than 512GB) and a sufficiently small VA size (e.g 39 bit), if the kernel
is loaded at the end of RAM we might not cover it in the linear mapping.

It would be nice if we could detect that and warn/stop if possible
(earlycon should be up by this point), rather than blowing up in strange
ways. The other option is to try to limit the memstart_addr such that we
know that we can map the kernel text (removing the unusable memory).

Mark.