Re: [Xen-devel] [PATCH v2 15/35] xen/arm64: head: Rework and document setup_fixmap()

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 22 Jul 2019, Julien Grall wrote:
> At the moment, the fixmap table is only hooked when earlyprintk is used.
> This is fine today because in C land, the fixmap is not used by anyone
> until the the boot CPU is switching to the runtime page-tables.
> 
> In the future, the boot CPU will not switch between page-tables to
> avoid TLB incoherency. Thus, the fixmap table will need to be always
> hooked beofre any use. Let's start doing it now in setup_fixmap().
         ^ before

Assuming you fix the typo:

Acked-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


> Lastly, document the behavior and the main registers usage within the
> function.
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> 
> ---
>     Changes in v2:
>         - Update the comment to reflect that we clobbers x1 - x4 and not
>         x0 - x1.
>         - Add the list of input registers
>         - s/ID map/1:1 mapping/
>         - Reword the commit message
> ---
>  xen/arch/arm/arm64/head.S | 18 ++++++++++++++++--
>  1 file changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
> index a607b3bdb1..f165dd61ca 100644
> --- a/xen/arch/arm/arm64/head.S
> +++ b/xen/arch/arm/arm64/head.S
> @@ -693,8 +693,21 @@ identity_mapping_removed:
>          ret
>  ENDPROC(remove_identity_mapping)
>  
> +/*
> + * Map the UART in the fixmap (when earlyprintk is used) and hook the
> + * fixmap table in the page tables.
> + *
> + * The fixmap cannot be mapped in create_page_tables because it may
> + * clash with the 1:1 mapping.
> + *
> + * Inputs:
> + *   x20: Physical offset
> + *   x23: Early UART base physical address
> + *
> + * Clobbers x1 - x4
> + */
>  setup_fixmap:
> -#if defined(CONFIG_EARLY_PRINTK) /* Fixmap is only used by early printk */
> +#ifdef CONFIG_EARLY_PRINTK
>          /* Add UART to the fixmap table */
>          ldr   x1, =xen_fixmap        /* x1 := vaddr (xen_fixmap) */
>          lsr   x2, x23, #THIRD_SHIFT
> @@ -702,6 +715,7 @@ setup_fixmap:
>          mov   x3, #PT_DEV_L3
>          orr   x2, x2, x3             /* x2 := 4K dev map including UART */
>          str   x2, [x1, #(FIXMAP_CONSOLE*8)] /* Map it in the first fixmap's 
> slot */
> +#endif
>  
>          /* Map fixmap into boot_second */
>          ldr   x4, =boot_second       /* x4 := vaddr (boot_second) */
> @@ -714,7 +728,7 @@ setup_fixmap:
>  
>          /* Ensure any page table updates made above have occurred. */
>          dsb   nshst
> -#endif
> +
>          ret
>  ENDPROC(setup_fixmap)
>  
> -- 
> 2.11.0
> 

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