Re: [Xen-devel] [PATCH v3 08/38] arm/p2m: Free p2m entries only in the hostp2m

[Julien Grall <julien.grall@xxxxxxx>]

Hello Sergej,

On 16/08/16 23:16, Sergej Proskurin wrote:
> Freeing p2m entries of arbitrary p2m's (in particular in alternate
> p2m's) will lead to unpredicted behavior as the entries might still be
> used within the host's p2m. The host's p2m should, however, free the
> entries, as it is the main instance responsible for their management. If
> entries were freed in the host's p2m, but still reside in one or more of
> the alternate p2m's, the change will be propagated to these functions as
> will be shown in the following commits.
>
> Signed-off-by: Sergej Proskurin <proskurin@xxxxxxxxxxxxx>
> ---
> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> Cc: Julien Grall <julien.grall@xxxxxxx>
> ---
>  xen/arch/arm/p2m.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> index 02e9ee7..bfbccca 100644
> --- a/xen/arch/arm/p2m.c
> +++ b/xen/arch/arm/p2m.c
> @@ -1004,7 +1004,9 @@ static int __p2m_set_entry(struct p2m_domain *p2m,
>       * Free the entry only if the original pte was valid and the base
>       * is different (to avoid freeing when permission is changed).
>       */
> -    if ( p2m_valid(orig_pte) && entry->p2m.base != orig_pte.p2m.base )
> +    if ( p2m_valid(orig_pte) &&
> +         entry->p2m.base != orig_pte.p2m.base &&
> +         p2m_is_hostp2m(p2m) )
With this change, intermediate page table will not be freed which will 
lead to keep the memory used (and therefore unavailable for others) 
until the p2m is effectively destroyed.
Regards,

--
Julien Grall

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