[PATCH] IOMMU: avoid double flushing in shared page table case

[Jan Beulich <jbeulich@xxxxxxxx>]

While the flush coalescing optimization has been helping the non-shared
case, it has actually lead to double flushes in the shared case (which
ought to be the more common one nowadays at least): Once from
*_set_entry() and a second time up the call tree from wherever the
overriding flag gets played with. In alignment with XSA-346 suppress
flushing in this case.

Similarly avoid excessive setting of IOMMU_FLUSHF_added on the batched
flushes: "idx" hasn't been added a new mapping for.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
TBD: The Arm part really is just for completeness (and hence could also
     be dropped) - the affected mapping spaces aren't currently
     supported there.

--- a/xen/arch/arm/p2m.c
+++ b/xen/arch/arm/p2m.c
@@ -1045,7 +1045,7 @@ static int __p2m_set_entry(struct p2m_do
         p2m->lowest_mapped_gfn = gfn_min(p2m->lowest_mapped_gfn, sgfn);
     }
 
-    if ( is_iommu_enabled(p2m->domain) &&
+    if ( is_iommu_enabled(p2m->domain) && !this_cpu(iommu_dont_flush_iotlb) &&
          (lpae_is_valid(orig_pte) || lpae_is_valid(*entry)) )
     {
         unsigned int flush_flags = 0;
--- a/xen/arch/x86/mm/p2m-ept.c
+++ b/xen/arch/x86/mm/p2m-ept.c
@@ -842,7 +842,7 @@ out:
     if ( rc == 0 && p2m_is_hostp2m(p2m) &&
          need_modify_vtd_table )
     {
-        if ( iommu_use_hap_pt(d) )
+        if ( iommu_use_hap_pt(d) && !this_cpu(iommu_dont_flush_iotlb) )
             rc = iommu_iotlb_flush(d, _dfn(gfn), 1ul << order,
                                    (iommu_flags ? IOMMU_FLUSHF_added : 0) |
                                    (vtd_pte_present ? IOMMU_FLUSHF_modified
--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -870,7 +870,7 @@ int xenmem_add_to_physmap(struct domain
         this_cpu(iommu_dont_flush_iotlb) = 0;
 
         ret = iommu_iotlb_flush(d, _dfn(xatp->idx - done), done,
-                                IOMMU_FLUSHF_added | IOMMU_FLUSHF_modified);
+                                IOMMU_FLUSHF_modified);
         if ( unlikely(ret) && rc >= 0 )
             rc = ret;