Re: [PATCH] xen/arm: mm: optimize setup_pagetables

[Julien Grall <julien.grall@arm.com>]

Hi Peng,

I would rename the title: "xen/arm: mm: remove unnecessary tlb flush in 
setup_pagetables".

On 11/05/2016 08:59, Peng Fan wrote:
> Before relocating xen to high address, need to build
> the mapping BOOT_RELOC_VIRT_START to xen_paddr into
> boot_pgtable and xen_pgtable.
>
> In setup_pagetables, relocate_xen will switch the root
> page table from boot_pgtable to xen_pgtable/cpu0_pgtable.
>
> Before relocate_xen:
> If touching xen_pgtable, no need to flush_xen_data_tlb_range_va_local.
> We only need to flush tlb when touch boot_pgtable.
> And no need to use write_pte for xen_pgtable, we can simply use
> "xen_second[second_table_offset(dest_va)] = pte;"
>
> Also move the piece code near DTB/Fixmap/XEN_VIRT_START to
> be more easier to understand.

The commit message is rather hard to understand. Can you please rework 
it say:
   - CPU0 is using the boot pages table and xen_second is not part of 
them => No need to flush the TLB
   - Clean up the code (i.e the code movement)

However, I would prefer to see 2 patches, one for removing the tlb 
flush, the other to move the code.

[...]

> diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> index 94ea054..bebd82f 100644
> --- a/xen/arch/arm/mm.c
> +++ b/xen/arch/arm/mm.c
> @@ -443,12 +443,6 @@ void __init setup_pagetables(unsigned long boot_phys_offset, paddr_t xen_paddr)
>       lpae_t pte, *p;
>       int i;
>
> -    /* Map the destination in the boot misc area. */
> -    dest_va = BOOT_RELOC_VIRT_START;
> -    pte = mfn_to_xen_entry(xen_paddr >> PAGE_SHIFT, WRITEALLOC);
> -    write_pte(xen_second + second_table_offset(dest_va), pte);
> -    flush_xen_data_tlb_range_va_local(dest_va, SECOND_SIZE);
> -
>       /* Calculate virt-to-phys offset for the new location */
>       phys_offset = xen_paddr - (unsigned long) _start;
>
> @@ -498,6 +492,11 @@ void __init setup_pagetables(unsigned long boot_phys_offset, paddr_t xen_paddr)
>       /* Map the destination in the boot misc area. */
>       dest_va = BOOT_RELOC_VIRT_START;
>       pte = mfn_to_xen_entry(xen_paddr >> PAGE_SHIFT, WRITEALLOC);
> +    xen_second[second_table_offset(dest_va)] = pte;
> +
> +    /* Map the destination in the boot misc area. */
> +    dest_va = BOOT_RELOC_VIRT_START;
> +    pte = mfn_to_xen_entry(xen_paddr >> PAGE_SHIFT, WRITEALLOC);

Why do you need to recompute "pte" and "dest_va"? They will be the same 
for "boot_second" and "xen_second".

Also, I think the code is still confusing in the current state because 
of the comments "Map the destination in the boot misc area.". They need 
to be updated to differentiate xen_second vs boot_second.

>       write_pte(boot_second + second_table_offset(dest_va), pte);
>       flush_xen_data_tlb_range_va_local(dest_va, SECOND_SIZE);
>   #ifdef CONFIG_ARM_64
>

Regards,

-- 
Julien Grall

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