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RE: [PATCH] xen/arm64: Fix incorrect DIRECTMAP_SIZE calculation
- To: Michal Orzel <michal.orzel@xxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Henry Wang <Henry.Wang@xxxxxxx>
- Date: Tue, 17 Jan 2023 03:10:28 +0000
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- Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Bertrand Marquis <Bertrand.Marquis@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
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- Thread-topic: [PATCH] xen/arm64: Fix incorrect DIRECTMAP_SIZE calculation
Hi Michal,
> -----Original Message-----
> Subject: [PATCH] xen/arm64: Fix incorrect DIRECTMAP_SIZE calculation
>
> The direct mapped area occupies L0 slots from 256 to 265 (i.e. 10 slots),
> resulting in 5TB (512GB * 10) of virtual address space. However, due to
> incorrect slot subtraction (we take 9 slots into account) we set
> DIRECTMAP_SIZE to 4.5TB instead. Fix it.
>
> Fixes: 5263507b1b4a ("xen: arm: Use a direct mapping of RAM on arm64")
> Signed-off-by: Michal Orzel <michal.orzel@xxxxxxx>
> ---
> xen/arch/arm/include/asm/config.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/xen/arch/arm/include/asm/config.h
> b/xen/arch/arm/include/asm/config.h
> index 0fefed1b8aa9..16213c8b677f 100644
> --- a/xen/arch/arm/include/asm/config.h
> +++ b/xen/arch/arm/include/asm/config.h
> @@ -157,7 +157,7 @@
> #define FRAMETABLE_NR (FRAMETABLE_SIZE / sizeof(*frame_table))
>
> #define DIRECTMAP_VIRT_START SLOT0(256)
> -#define DIRECTMAP_SIZE (SLOT0_ENTRY_SIZE * (265-256))
> +#define DIRECTMAP_SIZE (SLOT0_ENTRY_SIZE * (266 - 256))
>From the commit message "L0 slots from 256 to 265 (i.e. 10 slots)", I think
the actual range is [256, 265] so probably using "(265 - 256 + 1)" here is a
bit better? It seems to me that the number 266 looks like a magic number
because 266 is not in the range. But this is my personal taste though and I
am open to discussion if you or maintainers have other opinions.
Maybe we can also putting a comment on top of the macro to explain this
calculation.
I did test this patch on FVP using XTP in both arm32 and arm64 execution
mode, and this patch is good, so:
Tested-by: Henry Wang <Henry.Wang@xxxxxxx>
Kind regards,
Henry
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