[PATCH for-4.19] ppc/riscv: fix arch_acquire_resource_check()

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

None of the implementations support set_foreign_p2m_entry() yet, neither they
have a p2m walk in domain_relinquish_resources() in order to remove the foreign
mappings from the p2m and thus drop the extra refcounts.

Adjust the arch helpers to return false and introduce a comment that clearly
states it is not only taking extra refcounts that's needed, but also dropping
them on domain teardown.

Fixes: 4988704e00d8 ('xen/riscv: introduce p2m.h')
Fixes: 4a2f68f90930 ('xen/ppc: Define minimal stub headers required for full 
build')
Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/ppc/include/asm/p2m.h   | 7 ++++---
 xen/arch/riscv/include/asm/p2m.h | 7 ++++---
 2 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/xen/arch/ppc/include/asm/p2m.h b/xen/arch/ppc/include/asm/p2m.h
index 25ba05466853..f144ef8e1a54 100644
--- a/xen/arch/ppc/include/asm/p2m.h
+++ b/xen/arch/ppc/include/asm/p2m.h
@@ -81,10 +81,11 @@ static inline mfn_t gfn_to_mfn(struct domain *d, gfn_t gfn)
 static inline bool arch_acquire_resource_check(struct domain *d)
 {
     /*
-     * The reference counting of foreign entries in set_foreign_p2m_entry()
-     * is supported on PPC.
+     * Requires refcounting the foreign mappings and walking the p2m on
+     * teardown in order to remove foreign pages from the p2m and drop the
+     * extra reference counts.
      */
-    return true;
+    return false;
 }
 
 static inline void p2m_altp2m_check(struct vcpu *v, uint16_t idx)
diff --git a/xen/arch/riscv/include/asm/p2m.h b/xen/arch/riscv/include/asm/p2m.h
index 87b13f897926..387f372b5d26 100644
--- a/xen/arch/riscv/include/asm/p2m.h
+++ b/xen/arch/riscv/include/asm/p2m.h
@@ -79,10 +79,11 @@ static inline mfn_t gfn_to_mfn(struct domain *d, gfn_t gfn)
 static inline bool arch_acquire_resource_check(struct domain *d)
 {
     /*
-     * The reference counting of foreign entries in set_foreign_p2m_entry()
-     * is supported on RISCV.
+     * Requires refcounting the foreign mappings and walking the p2m on
+     * teardown in order to remove foreign pages from the p2m and drop the
+     * extra reference counts.
      */
-    return true;
+    return false;
 }
 
 static inline void p2m_altp2m_check(struct vcpu *v, uint16_t idx)
-- 
2.44.0