IOMMU: suppress "iommu_dont_flush_iotlb" when about to free a page

Deferring flushes to a single, wide range one - as is done when
handling XENMAPSPACE_gmfn_range - is okay only as long as
pages don't get freed ahead of the eventual flush. While the only
function setting the flag (xenmem_add_to_physmap()) suggests by its name
that it's only mapping new entries, in reality the way
xenmem_add_to_physmap_one() works means an unmap would happen not only
for the page being moved (but not freed) but, if the destination GFN is
populated, also for the page being displaced from that GFN. Collapsing
the two flushes for this GFN into just one (end even more so deferring
it to a batched invocation) is not correct.

This is part of XSA-346.

Fixes: cf95b2a9fd ("iommu: Introduce per cpu flag (iommu_dont_flush_iotlb) to avoid unnecessary iotlb... ")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Acked-by: Julien Grall <jgrall@amazon.com>
master commit: dea460d869
master date: 2020-10-20 14:21:09 +0200

xen/common/memory.c

@@ -298,7 +298,10 @@ int guest_remove_page(struct domain *d, unsigned long gmfn)
     p2m_type_t p2mt;
 #endif
     mfn_t mfn;
+#ifdef CONFIG_HAS_PASSTHROUGH
+    bool *dont_flush_p, dont_flush;
     int rc;
+#endif
 
 #ifdef CONFIG_X86
     mfn = get_gfn_query(d, gmfn, &p2mt);
@@ -376,8 +379,22 @@ int guest_remove_page(struct domain *d, unsigned long gmfn)
         return -ENXIO;
     }
 
+#ifdef CONFIG_HAS_PASSTHROUGH
+    /*
+     * Since we're likely to free the page below, we need to suspend
+     * xenmem_add_to_physmap()'s suppressing of IOMMU TLB flushes.
+     */
+    dont_flush_p = &this_cpu(iommu_dont_flush_iotlb);
+    dont_flush = *dont_flush_p;
+    *dont_flush_p = false;
+#endif
+
     rc = guest_physmap_remove_page(d, _gfn(gmfn), mfn, 0);
 
+#ifdef CONFIG_HAS_PASSTHROUGH
+    *dont_flush_p = dont_flush;
+#endif
+
     /*
      * With the lack of an IOMMU on some platforms, domains with DMA-capable
      * device must retrieve the same pfn when the hypercall populate_physmap