[PATCH v2] arm64: mm: increase VA range of identity map

[Catalin Marinas]

On Tue, Feb 24, 2015 at 05:08:23PM +0000, Ard Biesheuvel wrote:
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -387,6 +387,28 @@ __create_page_tables:
>  	mov	x0, x25				// idmap_pg_dir
>  	ldr	x3, =KERNEL_START
>  	add	x3, x3, x28			// __pa(KERNEL_START)
> +
> +#ifndef CONFIG_ARM64_VA_BITS_48
> +#define EXTRA_SHIFT	(PGDIR_SHIFT + PAGE_SHIFT - 3)
> +	/*
> +	 * If VA_BITS < 48, it may be too small to allow for an ID mapping to be
> +	 * created that covers system RAM if that is located sufficiently high
> +	 * in the physical address space. So for the ID map, use an extended
> +	 * virtual range in that case, by configuring an additional translation
> +	 * level.
> +	 */
> +	adrp	x5, KERNEL_END
> +	clz	x5, x5			// # of leading 0's in __pa(KERNEL_END)
> +	cmp	x5, #TCR_T0SZ(VA_BITS)	// VA_BITS sufficiently large?

I think we need some better comment here for people looking at this code
again in the future (including myself). So the VA bits needed to cover
KERNEL_END are calculated as (64 - clz(__pa(KERNEL_END))). T0SZ is
calculated as (64 - VA_BITS) which means that T0SZ is
clz(__pa(KERNEL_END)).

> +	b.ge	1f			// .. then skip additional level
> +
> +	adrp	x6, idmap_t0sz
> +	str	x5, [x6, #:lo12:idmap_t0sz]
> +
> +	create_table_entry x0, x3, EXTRA_SHIFT, PTRS_PER_PGD, x5, x6

EXTRA_SHIFT is correctly calculated (one level more than PGDIR_SHIFT)
but I don't think PTRS_PER_PGD is appropriate here. For example, we had
in the past 39-bit VA with 64KB pages which made PTRS_PER_PGD pretty
small (well, it may cover the 40-bit case you are trying to fix but not
up to 48). So maybe define something like:

#define EXTRA_PTRS	(1 << (48 - EXTRA_SHIFT))

Otherwise the patch looks fine to me. We have to look into fixing KVM as
well, ideally with sharing the same idmap with the kernel.

-- 
Catalin