Re: [PATCH v3 07/19] xen/arm: mm: Don't open-code Xen PT update in remove_early_mappings()

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 21 Feb 2022, Julien Grall wrote:
> From: Julien Grall <julien.grall@xxxxxxx>
> 
> Now that xen_pt_update_entry() is able to deal with different mapping
> size, we can replace the open-coding of the page-tables update by a call
> to modify_xen_mappings().
> 
> As the function is not meant to fail, a BUG_ON() is added to check the
> return.
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>

Nice!


> ---
>     Changes in v2:
>         - Stay consistent with how function name are used in the commit
>         message
>         - Add my AWS signed-off-by
> ---
>  xen/arch/arm/mm.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> index 7b4b9de8693e..f088a4b2de96 100644
> --- a/xen/arch/arm/mm.c
> +++ b/xen/arch/arm/mm.c
> @@ -599,11 +599,11 @@ void * __init early_fdt_map(paddr_t fdt_paddr)
>  
>  void __init remove_early_mappings(void)
>  {
> -    lpae_t pte = {0};
> -    write_pte(xen_second + second_table_offset(BOOT_FDT_VIRT_START), pte);
> -    write_pte(xen_second + second_table_offset(BOOT_FDT_VIRT_START + SZ_2M),
> -              pte);
> -    flush_xen_tlb_range_va(BOOT_FDT_VIRT_START, BOOT_FDT_SLOT_SIZE);
> +    int rc;
> +
> +    rc = modify_xen_mappings(BOOT_FDT_VIRT_START, BOOT_FDT_VIRT_END,
> +                             _PAGE_BLOCK);
> +    BUG_ON(rc);

Am I right that we are actually destroying the mapping, which usually is
done by calling destroy_xen_mappings, but we cannot call
destroy_xen_mappings in this case because it doesn't take a flags
parameter?

If so, then I would add a flags parameter to destroy_xen_mappings
instead of calling modify_xen_mappings just to pass _PAGE_BLOCK.
But I don't feel strongly about it so if you don't feel like making the
change to destroy_xen_mappings, you can add my acked-by here anyway.