Re: [PATCH v2 REPOST 02/12] x86/mm: allow a privileged PV domain to map guest mfns

[Wei Liu <wei.liu2@citrix.com>]

On Tue, Aug 22, 2017 at 03:50:56PM +0100, Paul Durrant wrote:
> In the case where a PV domain is mapping guest resources then it needs make
> the HYPERVISOR_mmu_update call using DOMID_SELF, rather than the guest
> domid, so that the passed in gmfn values are correctly treated as mfns
> rather than gfns present in the guest p2m.
> 

What would be the callchain like in this case?

I don't quite understand how this fits with the resource mapping code
in this series.

> This patch removes a check which currently disallows mapping of a page when
> the owner of the page tables matches the domain passed to
> HYPERVISOR_mmu_update, but that domain is not the real owner of the page.
> The check was introduced by patch d3c6a215ca9 ("x86: Clean up
> get_page_from_l1e() to correctly distinguish between owner-of-pte and
> owner-of-data-page in all cases") but it's not clear why it was needed.
> 
> Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
> ---
> Cc: Jan Beulich <jbeulich@suse.com>
> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
> ---
>  xen/arch/x86/mm.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
> index 0abb1e284f..aaa9ff5197 100644
> --- a/xen/arch/x86/mm.c
> +++ b/xen/arch/x86/mm.c
> @@ -989,12 +989,15 @@ get_page_from_l1e(
>                     (real_pg_owner != dom_cow) ) )
>      {
>          /*
> -         * Let privileged domains transfer the right to map their target
> -         * domain's pages. This is used to allow stub-domain pvfb export to
> -         * dom0, until pvfb supports granted mappings. At that time this
> -         * minor hack can go away.
> +         * If the real page owner is not the domain specified in the
> +         * hypercall then establish that the specified domain has
> +         * mapping privilege over the page owner.
> +         * This is used to allow stub-domain pvfb export to dom0. It is
> +         * also used to allow a privileged PV domain to map mfns using
> +         * DOMID_SELF, which is needed for mapping guest resources such
> +         * grant table frames.
>           */
> -        if ( (real_pg_owner == NULL) || (pg_owner == l1e_owner) ||
> +        if ( (real_pg_owner == NULL) ||
>               xsm_priv_mapping(XSM_TARGET, pg_owner, real_pg_owner) )
>          {
>              gdprintk(XENLOG_WARNING,
> -- 
> 2.11.0
> 
> 
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