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KVM: Fix x86 emulator writeback

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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>
This commit is contained in:
Luca Tettamanti 2007-06-19 22:41:20 +02:00 committed by Avi Kivity
parent 74906345ff
commit 02c03a326a
1 changed files with 5 additions and 4 deletions
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9
drivers/kvm/x86_emulate.c
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@ -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;