Re: [Xen-devel] [PATCH v2] xen: arm: avoid reusing incorrect mappings when walking the p2m.

[Julien Grall <julien.grall@xxxxxxxxxx>]

Hi Ian,

On 18/07/14 14:33, Ian Campbell wrote:
> When we change which PT page we are mapping at a given level then we need to
> invalidate any cached mappings further down the tree, otherwise we risk using
> them because their offset might match but be based on a different offset
> further up the table.
>
> e.g. when remapping first then cur_first_offset and cur_second_offset (which
> indicate the currently mapped second and third tables respectively) both become
> invalid
>
> Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
Acked-by: Julien Grall <julien.grall@xxxxxxxxxx>

Regards,

> ---
> v2: Also invalidate cur_first_offset when changing cur_first_page.
>      Corrected/clarified commit message, this issue isn't really specific to
>      superpages, they just happen to expose it.
> ---
>   xen/arch/arm/p2m.c |    4 ++++
>   1 file changed, 4 insertions(+)
>
> diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> index 2f855b5..8ffddac 100644
> --- a/xen/arch/arm/p2m.c
> +++ b/xen/arch/arm/p2m.c
> @@ -743,6 +743,8 @@ static int apply_p2m_changes(struct domain *d,
>                   goto out;
>               }
>               cur_first_page = p2m_first_level_index(addr);
> +            /* Any mapping further down is now invalid */
> +            cur_first_offset = cur_second_offset = ~0;
>           }
>
>           /* We only use a 3 level p2m at the moment, so no level 0,
> @@ -765,6 +767,8 @@ static int apply_p2m_changes(struct domain *d,
>               if (second) unmap_domain_page(second);
>               second = 
> map_domain_page(first[first_table_offset(addr)].p2m.base);
>               cur_first_offset = first_table_offset(addr);
> +            /* Any mapping further down is now invalid */
> +            cur_second_offset = ~0;
>           }
>           /* else: second already valid */
--
Julien Grall

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