Re: [Xen-devel] [PATCH 15/17] xen/arm64: head: Rework and document setup_fixmap()

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 10 Jun 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 conflict. This means the fixmap table will need to be hooked before
> any use. For simplicity, setup_fixmap() will now do that job.

Can I ask you to reword this slightly, especially the last sentence? It
took me a while to understand what you meant. I suggest:

 In the future, the boot CPU will not switch between page-tables to
 avoid any TLB conflicts. Thus, the fixmap table will need to be always
 hooked before any use. Let's start doing it now in setup_fixmap().

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>
> ---
>  xen/arch/arm/arm64/head.S | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
> index 96e85f8834..4f7fa6769f 100644
> --- a/xen/arch/arm/arm64/head.S
> +++ b/xen/arch/arm/arm64/head.S
> @@ -700,8 +700,17 @@ id_map_removed:
>          ret
>  ENDPROC(remove_id_map)
>  
> +/*
> + * 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 ID map.
> + *
> + * Clobbers x0 - x1
> + */
>  setup_fixmap:
> -#if defined(CONFIG_EARLY_PRINTK) /* Fixmap is only used by early printk */
> +#ifdef CONFIG_EARLY_PRINTK

I am curious why you made this code style change


>          /* Add UART to the fixmap table */
>          ldr   x1, =xen_fixmap        /* x1 := vaddr (xen_fixmap) */
>          lsr   x2, x23, #THIRD_SHIFT
> @@ -709,6 +718,7 @@ 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) */
> @@ -721,7 +731,6 @@ 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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