[PATCH] Documentation: arm64: Update memory.rst for TBI

Mark Rutland mark.rutland at arm.com
Thu Jul 4 03:36:41 PDT 2024


On Tue, Jul 02, 2024 at 10:13:49AM +0100, Kevin Brodsky wrote:
> Most of memory.rst was written very early, at a time where TBI (Top
> Byte Ignore) was not enabled. Nowadays TBI0 is always enabled, and
> TBI1 may be enabled, depending on the kernel configuration. This
> means that VA bits 63:56 cannot generally be assumed to have any
> particular value.
> 
> Regardless of TBI, TTBRx selection is done based on bit 55; update
> memory.rst accordingly.
> 
> Signed-off-by: Kevin Brodsky <kevin.brodsky at arm.com>

Acked-by: Mark Rutland <mark.rutland at arm.com>

Mark.

> ---
>  Documentation/arch/arm64/memory.rst | 42 ++++++++++++++---------------
>  1 file changed, 20 insertions(+), 22 deletions(-)
> 
> diff --git a/Documentation/arch/arm64/memory.rst b/Documentation/arch/arm64/memory.rst
> index 55a55f30eed8..8a658984b8bb 100644
> --- a/Documentation/arch/arm64/memory.rst
> +++ b/Documentation/arch/arm64/memory.rst
> @@ -18,12 +18,10 @@ ARMv8.2 adds optional support for Large Virtual Address space. This is
>  only available when running with a 64KB page size and expands the
>  number of descriptors in the first level of translation.
>  
> -User addresses have bits 63:48 set to 0 while the kernel addresses have
> -the same bits set to 1. TTBRx selection is given by bit 63 of the
> -virtual address. The swapper_pg_dir contains only kernel (global)
> -mappings while the user pgd contains only user (non-global) mappings.
> -The swapper_pg_dir address is written to TTBR1 and never written to
> -TTBR0.
> +TTBRx selection is given by bit 55 of the virtual address. The
> +swapper_pg_dir contains only kernel (global) mappings while the user pgd
> +contains only user (non-global) mappings.  The swapper_pg_dir address is
> +written to TTBR1 and never written to TTBR0.
>  
>  
>  AArch64 Linux memory layout with 4KB pages + 4 levels (48-bit)::
> @@ -65,14 +63,14 @@ Translation table lookup with 4KB pages::
>    +--------+--------+--------+--------+--------+--------+--------+--------+
>    |63    56|55    48|47    40|39    32|31    24|23    16|15     8|7      0|
>    +--------+--------+--------+--------+--------+--------+--------+--------+
> -   |                 |         |         |         |         |
> -   |                 |         |         |         |         v
> -   |                 |         |         |         |   [11:0]  in-page offset
> -   |                 |         |         |         +-> [20:12] L3 index
> -   |                 |         |         +-----------> [29:21] L2 index
> -   |                 |         +---------------------> [38:30] L1 index
> -   |                 +-------------------------------> [47:39] L0 index
> -   +-------------------------------------------------> [63] TTBR0/1
> +            |        |         |         |         |         |
> +            |        |         |         |         |         v
> +            |        |         |         |         |   [11:0]  in-page offset
> +            |        |         |         |         +-> [20:12] L3 index
> +            |        |         |         +-----------> [29:21] L2 index
> +            |        |         +---------------------> [38:30] L1 index
> +            |        +-------------------------------> [47:39] L0 index
> +            +----------------------------------------> [55] TTBR0/1
>  
>  
>  Translation table lookup with 64KB pages::
> @@ -80,14 +78,14 @@ Translation table lookup with 64KB pages::
>    +--------+--------+--------+--------+--------+--------+--------+--------+
>    |63    56|55    48|47    40|39    32|31    24|23    16|15     8|7      0|
>    +--------+--------+--------+--------+--------+--------+--------+--------+
> -   |                 |    |               |              |
> -   |                 |    |               |              v
> -   |                 |    |               |            [15:0]  in-page offset
> -   |                 |    |               +----------> [28:16] L3 index
> -   |                 |    +--------------------------> [41:29] L2 index
> -   |                 +-------------------------------> [47:42] L1 index (48-bit)
> -   |                                                   [51:42] L1 index (52-bit)
> -   +-------------------------------------------------> [63] TTBR0/1
> +            |        |    |               |              |
> +            |        |    |               |              v
> +            |        |    |               |            [15:0]  in-page offset
> +            |        |    |               +----------> [28:16] L3 index
> +            |        |    +--------------------------> [41:29] L2 index
> +            |        +-------------------------------> [47:42] L1 index (48-bit)
> +            |                                          [51:42] L1 index (52-bit)
> +            +----------------------------------------> [55] TTBR0/1
>  
>  
>  When using KVM without the Virtualization Host Extensions, the
> -- 
> 2.43.0
> 



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