[Xen-devel] [PATCH] clflush instruction emulation

[Wei Huang <wei.huang2@xxxxxxx>]

We recently found that FreeBSD 8.0 guest failed to install and boot on 
Xen. The reason was that FreeBSD detected clflush feature and invoked 
this instruction to flush MMIO space. This caused a page fault; but 
x86_emulate.c failed to emulate this instruction (not supported). As a 
result, a page fault was detected inside FreeBSD. A similar issue was 
reported earlier.
http://lists.xensource.com/archives/html/xen-devel/2010-03/msg00362.html

I created a patch which enables clflush emulation. I have verified that 
this patch solves FreeBSD issue. This patch returns immediately without 
doing anything. Note that we don't have to intercept clflush in SVM. So 
the only time we have page_fault for clflush instruction is when guest 
VM flushes MMIO space. In this case, do we need to send this command 
over to QEMU? I didn't do it in this patch anyway.
Best,
-Wei


diff -r 12a610b600b0 xen/arch/x86/x86_emulate/x86_emulate.c
--- a/xen/arch/x86/x86_emulate/x86_emulate.c    Wed Apr 14 11:50:00 2010 -0500
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c    Wed Apr 14 16:11:36 2010 -0500
@@ -227,7 +227,8 @@
     DstMem|SrcReg|ModRM, DstMem|SrcReg|ModRM, 0, 0,
     /* 0xA8 - 0xAF */
     ImplicitOps, ImplicitOps, 0, DstBitBase|SrcReg|ModRM,
-    DstMem|SrcReg|ModRM, DstMem|SrcReg|ModRM, 0, DstReg|SrcMem|ModRM,
+    DstMem|SrcReg|ModRM, DstMem|SrcReg|ModRM, DstMem|SrcReg|ModRM, 
+    DstReg|SrcMem|ModRM,
     /* 0xB0 - 0xB7 */
     ByteOp|DstMem|SrcReg|ModRM, DstMem|SrcReg|ModRM,
     DstReg|SrcMem|ModRM|Mov, DstBitBase|SrcReg|ModRM,
@@ -3948,6 +3949,11 @@
         src.val = x86_seg_gs;
         goto pop_seg;
 
+    case 0xae: /* clflush mem8 */
+        /* we don't need to do anything here */
+        rc = X86EMUL_OKAY;
+        break;
+
     case 0xb0 ... 0xb1: /* cmpxchg */
         /* Save real source value, then compare EAX against destination. */
         src.orig_val = src.val;

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