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authorNadav Amit <namit@cs.technion.ac.il>2015-04-28 13:06:01 +0300
committerPaolo Bonzini <pbonzini@redhat.com>2015-05-19 20:52:36 +0200
commit428e3d08574b77876ea5e71f294f91bd8afa51b5 (patch)
tree88b926274e851fc4919468fdd24a30ca52086b2f
parentee122a7109e42313caadf6038ab773d1f68fcce1 (diff)
KVM: x86: Fix zero iterations REP-string
When a REP-string is executed in 64-bit mode with an address-size prefix, ECX/EDI/ESI are used as counter and pointers. When ECX is initially zero, Intel CPUs clear the high 32-bits of RCX, and recent Intel CPUs update the high bits of the pointers in MOVS/STOS. This behavior is specific to Intel according to few experiments. As one may guess, this is an undocumented behavior. Yet, it is observable in the guest, since at least VMX traps REP-INS/OUTS even when ECX=0. Note that VMware appears to get it right. The behavior can be observed using the following code: #include <stdio.h> #define LOW_MASK (0xffffffff00000000ull) #define ALL_MASK (0xffffffffffffffffull) #define TEST(opcode) \ do { \ asm volatile(".byte 0xf2 \n\t .byte 0x67 \n\t .byte " opcode "\n\t" \ : "=S"(s), "=c"(c), "=D"(d) \ : "S"(ALL_MASK), "c"(LOW_MASK), "D"(ALL_MASK)); \ printf("opcode %s rcx=%llx rsi=%llx rdi=%llx\n", \ opcode, c, s, d); \ } while(0) void main() { unsigned long long s, d, c; iopl(3); TEST("0x6c"); TEST("0x6d"); TEST("0x6e"); TEST("0x6f"); TEST("0xa4"); TEST("0xa5"); TEST("0xa6"); TEST("0xa7"); TEST("0xaa"); TEST("0xab"); TEST("0xae"); TEST("0xaf"); } Signed-off-by: Nadav Amit <namit@cs.technion.ac.il> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
-rw-r--r--arch/x86/kvm/emulate.c25
1 files changed, 25 insertions, 0 deletions
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index e8c03be83e48..9b655d113fc6 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -2570,6 +2570,30 @@ static bool emulator_io_permited(struct x86_emulate_ctxt *ctxt,
return true;
}
+static void string_registers_quirk(struct x86_emulate_ctxt *ctxt)
+{
+ /*
+ * Intel CPUs mask the counter and pointers in quite strange
+ * manner when ECX is zero due to REP-string optimizations.
+ */
+#ifdef CONFIG_X86_64
+ if (ctxt->ad_bytes != 4 || !vendor_intel(ctxt))
+ return;
+
+ *reg_write(ctxt, VCPU_REGS_RCX) = 0;
+
+ switch (ctxt->b) {
+ case 0xa4: /* movsb */
+ case 0xa5: /* movsd/w */
+ *reg_rmw(ctxt, VCPU_REGS_RSI) &= (u32)-1;
+ /* fall through */
+ case 0xaa: /* stosb */
+ case 0xab: /* stosd/w */
+ *reg_rmw(ctxt, VCPU_REGS_RDI) &= (u32)-1;
+ }
+#endif
+}
+
static void save_state_to_tss16(struct x86_emulate_ctxt *ctxt,
struct tss_segment_16 *tss)
{
@@ -4910,6 +4934,7 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
if (ctxt->rep_prefix && (ctxt->d & String)) {
/* All REP prefixes have the same first termination condition */
if (address_mask(ctxt, reg_read(ctxt, VCPU_REGS_RCX)) == 0) {
+ string_registers_quirk(ctxt);
ctxt->eip = ctxt->_eip;
ctxt->eflags &= ~X86_EFLAGS_RF;
goto done;