Re: [PATCH V4 16/24] xen/mm: Handle properly reference in set_foreign_p2m_entry() on Arm

[Julien Grall <julien@xxxxxxx>]

Hi Oleksandr,

On 12/01/2021 21:52, Oleksandr Tyshchenko wrote:
> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
>
> This patch implements reference counting of foreign entries in
> in set_foreign_p2m_entry() on Arm. This is a mandatory action if
> we want to run emulator (IOREQ server) in other than dom0 domain,
> as we can't trust it to do the right thing if it is not running
> in dom0. So we need to grab a reference on the page to avoid it
> disappearing.
>
> It is valid to always pass "p2m_map_foreign_rw" type to
> guest_physmap_add_entry() since the current and foreign domains
> would be always different. A case when they are equal would be
> rejected by rcu_lock_remote_domain_by_id(). Besides the similar
> comment in the code put a respective ASSERT() to catch incorrect
> usage in future.
>
> It was tested with IOREQ feature to confirm that all the pages given
> to this function belong to a domain, so we can use the same approach
> as for XENMAPSPACE_gmfn_foreign handling in xenmem_add_to_physmap_one().
>
> This involves adding an extra parameter for the foreign domain to
> set_foreign_p2m_entry() and a helper to indicate whether the arch
> supports the reference counting of foreign entries and the restriction
> for the hardware domain in the common code can be skipped for it.
>
> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> CC: Julien Grall <julien.grall@xxxxxxx>
> [On Arm only]
> Tested-by: Wei Chen <Wei.Chen@xxxxxxx>
Reviewed-by: Julien Grall <jgrall@xxxxxxxxxx>

Cheers,

--
Julien Grall