AMD/IOMMU: suppress PTE merging after initial table creation

The logic is not fit for this purpose, so simply disable its use until
it can be fixed / replaced. Note that this re-enables merging for the
table creation case, which was disabled as a (perhaps unintended) side
effect of the earlier "amd/iommu: fix flush checks". It relies on no
page getting mapped more than once (with different properties) in this
process, as that would still be beyond what the merging logic can cope
with. But arch_iommu_populate_page_table() guarantees this afaict.

This is part of XSA-275.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
master commit: 937ef32565
master date: 2018-11-20 14:55:14 +0100

xen/common/domain.c

@@ -1023,6 +1023,20 @@ int domain_unpause_by_systemcontroller(struct domain *d)
         prev = cmpxchg(&d->controller_pause_count, old, new);
     } while ( prev != old );
 
+    /*
+     * d->controller_pause_count is initialised to 1, and the toolstack is
+     * responsible for making one unpause hypercall when it wishes the guest
+     * to start running.
+     *
+     * All other toolstack operations should make a pair of pause/unpause
+     * calls and rely on the reference counting here.
+     *
+     * Creation is considered finished when the controller reference count
+     * first drops to 0.
+     */
+    if ( new == 0 )
+        d->creation_finished = 1;
+
     domain_unpause(d);
 
     return 0;

xen/drivers/passthrough/amd/iommu_map.c

@@ -695,11 +695,24 @@ int amd_iommu_map_page(struct domain *d, unsigned long gfn, unsigned long mfn,
                                        !!(flags & IOMMUF_writable),
                                        !!(flags & IOMMUF_readable));
 
-    /* Do not increase pde count if io mapping has not been changed */
-    if ( !need_flush )
-        goto out;
+    if ( need_flush )
+    {
+        amd_iommu_flush_pages(d, gfn, 0);
+        /* No further merging, as the logic doesn't cope. */
+        hd->arch.no_merge = 1;
+    }
 
-    amd_iommu_flush_pages(d, gfn, 0);
+    /*
+     * Suppress merging of non-R/W mappings or after initial table creation,
+     * as the merge logic does not cope with this.
+     */
+    if ( hd->arch.no_merge || flags != (IOMMUF_writable | IOMMUF_readable) )
+        goto out;
+    if ( d->creation_finished )
+    {
+        hd->arch.no_merge = 1;
+        goto out;
+    }
 
     for ( merge_level = IOMMU_PAGING_MODE_LEVEL_2;
           merge_level <= hd->arch.paging_mode; merge_level++ )
@@ -769,6 +782,10 @@ int amd_iommu_unmap_page(struct domain *d, unsigned long gfn)
 
     /* mark PTE as 'page not present' */
     clear_iommu_pte_present(pt_mfn[1], gfn);
+
+    /* No further merging in amd_iommu_map_page(), as the logic doesn't cope. */
+    hd->arch.no_merge = 1;
+
     spin_unlock(&hd->arch.mapping_lock);
 
     amd_iommu_flush_pages(d, gfn, 0);

xen/include/asm-x86/hvm/iommu.h

@@ -59,6 +59,7 @@ struct arch_iommu
 
     /* amd iommu support */
     int paging_mode;
+    bool_t no_merge;
     struct page_info *root_table;
     struct guest_iommu *g_iommu;
 };

xen/include/xen/sched.h

@@ -387,6 +387,12 @@ struct domain
     bool_t           disable_migrate;
     /* Is this guest being debugged by dom0? */
     bool_t           debugger_attached;
+    /*
+     * Set to true at the very end of domain creation, when the domain is
+     * unpaused for the first time by the systemcontroller.
+     */
+    bool_t           creation_finished;
+
     /* Which guest this guest has privileges on */
     struct domain   *target;