[Xen-changelog] [xen master] x86/HVM: use single (atomic) MOV for aligned emulated writes

[patchbot@xxxxxxx]

commit bf08a8a08a2ee8cef5fb7b3b274b0e09123a41bd
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Fri Jan 3 17:04:41 2020 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Jan 3 17:04:41 2020 +0100

    x86/HVM: use single (atomic) MOV for aligned emulated writes
    
    Using memcpy() may result in multiple individual byte accesses
    (dependening how memcpy() is implemented and how the resulting insns,
    e.g. REP MOVSB, get carried out in hardware), which isn't what we
    want/need for carrying out guest insns as correctly as possible. Fall
    back to memcpy() only for accesses not 2, 4, or 8 bytes in size.
    
    Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/hvm/emulate.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/hvm/emulate.c b/xen/arch/x86/hvm/emulate.c
index 637034b6a1..a3aa33a44f 100644
--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -1342,7 +1342,14 @@ static int hvmemul_write(
     if ( !mapping )
         return linear_write(addr, bytes, p_data, pfec, hvmemul_ctxt);
 
-    memcpy(mapping, p_data, bytes);
+    /* Where possible use single (and hence generally atomic) MOV insns. */
+    switch ( bytes )
+    {
+    case 2: write_u16_atomic(mapping, *(uint16_t *)p_data); break;
+    case 4: write_u32_atomic(mapping, *(uint32_t *)p_data); break;
+    case 8: write_u64_atomic(mapping, *(uint64_t *)p_data); break;
+    default: memcpy(mapping, p_data, bytes);                break;
+    }
 
     hvmemul_unmap_linear_addr(mapping, addr, bytes, hvmemul_ctxt);
 
--
generated by git-patchbot for /home/xen/git/xen.git#master

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