KVM: Fix x86 emulator writeback

When the old value and new one are the same the emulator skips the
write; this is undesirable when the destination is a MMIO area and the
write shall be performed regardless of the previous value. This
optimization breaks e.g. a Linux guest APIC compiled without
X86_GOOD_APIC.

Remove the check and perform the writeback stage in the emulation unless
it's explicitly disabled (currently push and some 2 bytes instructions
may disable the writeback).

Signed-Off-By: Luca Tettamanti <kronos.it@gmail.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>

1 files changed, 5 insertions, 4 deletions

diff --git a/drivers/kvm/x86_emulate.c b/drivers/kvm/x86_emulate.c
index 92620e48f06d..f60012d62610 100644
--- a/drivers/kvm/x86_emulate.c
+++ b/drivers/kvm/x86_emulate.c
@@ -485,6 +485,7 @@ x86_emulate_memop(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
         int mode = ctxt->mode;
         unsigned long modrm_ea;
         int use_modrm_ea, index_reg = 0, base_reg = 0, scale, rip_relative = 0;
+        int no_wb = 0;
 
         /* Shadow copy of register state. Committed on successful emulation. */
         unsigned long _regs[NR_VCPU_REGS];
@@ -1051,7 +1052,7 @@ done_prefixes:
                                                       _regs[VCPU_REGS_RSP]),
                                      &dst.val, dst.bytes, ctxt)) != 0)
                                 goto done;
-                        dst.val = dst.orig_val; /* skanky: disable writeback */
+                        no_wb = 1;
                         break;
                 default:
                         goto cannot_emulate;
@@ -1060,7 +1061,7 @@ done_prefixes:
         }
 
 writeback:
-        if ((d & Mov) || (dst.orig_val != dst.val)) {
+        if (!no_wb) {
                 switch (dst.type) {
                 case OP_REG:
                         /* The 4-byte case *is* correct: in 64-bit mode we zero-extend. */
@@ -1168,7 +1169,7 @@ pop_instruction:
                         goto done;
 
                 register_address_increment(_regs[VCPU_REGS_RSP], op_bytes);
-                dst.orig_val = dst.val; /* Disable writeback. */
+                no_wb = 1; /* Disable writeback. */
                 break;
         }
         goto writeback;
@@ -1323,7 +1324,7 @@ twobyte_insn:
 
 twobyte_special_insn:
         /* Disable writeback. */
-        dst.orig_val = dst.val;
+        no_wb = 1;
         switch (b) {
         case 0x09:              /* wbinvd */
                 break;