[Xen-changelog] [xen staging] x86/mm: avoid IOMMU operations in more cases in _get_page_type()

[patchbot@xxxxxxx]

commit c497bfa63390db9d2242c1ae2dfbee2a000e7ece
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Fri Dec 27 10:01:43 2019 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Dec 27 10:01:43 2019 +0100

    x86/mm: avoid IOMMU operations in more cases in _get_page_type()
    
    All that really matters is whether writability of a page changes; in
    particular e.g. page table -> page table (but different levels)
    transitions do not require unmapping the page from the IOMMU again.
    
    Note that the XSA-288 fix did arrange for PGT_none pages not needing
    special consideration here.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/mm.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 594f1bd290..b48777cc19 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -2980,17 +2980,19 @@ static int _get_page_type(struct page_info *page, 
unsigned long type,
             return -EINTR;
     }
 
-    if ( unlikely((x & PGT_type_mask) != type) )
+    if ( unlikely(((x & PGT_type_mask) == PGT_writable_page) !=
+                  (type == PGT_writable_page)) )
     {
         /* Special pages should not be accessible from devices. */
         struct domain *d = page_get_owner(page);
+
         if ( d && is_pv_domain(d) && unlikely(need_iommu_pt_sync(d)) )
         {
             mfn_t mfn = page_to_mfn(page);
 
             if ( (x & PGT_type_mask) == PGT_writable_page )
                 rc = iommu_legacy_unmap(d, _dfn(mfn_x(mfn)), PAGE_ORDER_4K);
-            else if ( type == PGT_writable_page )
+            else
                 rc = iommu_legacy_map(d, _dfn(mfn_x(mfn)), mfn, PAGE_ORDER_4K,
                                       IOMMUF_readable | IOMMUF_writable);
 
--
generated by git-patchbot for /home/xen/git/xen.git#staging

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