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

[patchbot@xxxxxxx]

commit 32772fbb3cf7498817304b53b087e325c6991716
Author:     Jason Andryuk <jandryuk@xxxxxxxxx>
AuthorDate: Fri Jan 17 16:19:16 2020 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Jan 17 16:19:16 2020 +0100

    x86/shadow: use single (atomic) MOV for emulated writes
    
    This is the corresponding change to the shadow code as made by
    bf08a8a08a2e "x86/HVM: use single (atomic) MOV for aligned emulated
    writes" to the non-shadow HVM code.
    
    The bf08a8a08a2e commit message:
    Using memcpy() may result in multiple individual byte accesses
    (depending 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.
    
    Signed-off-by: Jason Andryuk <jandryuk@xxxxxxxxx>
    Acked-by: Tim Deegan <tim@xxxxxxx>
---
 xen/arch/x86/mm/shadow/hvm.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/mm/shadow/hvm.c b/xen/arch/x86/mm/shadow/hvm.c
index 48dfad4557..a219266fa2 100644
--- a/xen/arch/x86/mm/shadow/hvm.c
+++ b/xen/arch/x86/mm/shadow/hvm.c
@@ -215,7 +215,15 @@ hvm_emulate_write(enum x86_segment seg,
         return ~PTR_ERR(ptr);
 
     paging_lock(v->domain);
-    memcpy(ptr, p_data, bytes);
+
+    /* Where possible use single (and hence generally atomic) MOV insns. */
+    switch ( bytes )
+    {
+    case 2: write_u16_atomic(ptr, *(uint16_t *)p_data); break;
+    case 4: write_u32_atomic(ptr, *(uint32_t *)p_data); break;
+    case 8: write_u64_atomic(ptr, *(uint64_t *)p_data); break;
+    default: memcpy(ptr, p_data, bytes);                break;
+    }
 
     if ( tb_init_done )
         v->arch.paging.mode->shadow.trace_emul_write_val(ptr, addr,
--
generated by git-patchbot for /home/xen/git/xen.git#master

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