@@ -962,6 +962,22 @@ FASTOP2(xadd);
FASTOP2R(cmp, cmp_r);
+static int em_bsf_c(struct x86_emulate_ctxt *ctxt)
+{
+ /* If src is zero, do not writeback, but update flags */
+ if (ctxt->src.val == 0)
+ ctxt->dst.type = OP_NONE;
+ return fastop(ctxt, em_bsf);
+}
+
+static int em_bsr_c(struct x86_emulate_ctxt *ctxt)
+{
+ /* If src is zero, do not writeback, but update flags */
+ if (ctxt->src.val == 0)
+ ctxt->dst.type = OP_NONE;
+ return fastop(ctxt, em_bsr);
+}
+
static u8 test_cc(unsigned int condition, unsigned long flags)
{
u8 rc;
@@ -4188,7 +4204,8 @@ static const struct opcode twobyte_table[256] = {
N, N,
G(BitOp, group8),
F(DstMem | SrcReg | ModRM | BitOp | Lock | PageTable, em_btc),
- F(DstReg | SrcMem | ModRM, em_bsf), F(DstReg | SrcMem | ModRM, em_bsr),
+ I(DstReg | SrcMem | ModRM, em_bsf_c),
+ I(DstReg | SrcMem | ModRM, em_bsr_c),
D(DstReg | SrcMem8 | ModRM | Mov), D(DstReg | SrcMem16 | ModRM | Mov),
/* 0xC0 - 0xC7 */
F2bv(DstMem | SrcReg | ModRM | SrcWrite | Lock, em_xadd),
If the source of BSF and BSR is zero, the destination register should not change. That is how real hardware behaves. If we set the destination even with the same value that we had before, we may clear bits [63:32] unnecassarily. Signed-off-by: Nadav Amit <namit@cs.technion.ac.il> --- arch/x86/kvm/emulate.c | 19 ++++++++++++++++++- 1 file changed, 18 insertions(+), 1 deletion(-)