KVM: x86 emulator: restart string instruction without going back to a guest.

Currently when string instruction is only partially complete we go back
to a guest mode, guest tries to reexecute instruction and exits again
and at this point emulation continues. Avoid all of this by restarting
instruction without going back to a guest mode, but return to a guest
mode each 1024 iterations to allow interrupt injection. Pending
exception causes immediate guest entry too.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

1 files changed, 23 insertions, 11 deletions

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 0579d9dd9aac..6de6ad1610d8 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -927,8 +927,11 @@ x86_decode_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
         int mode = ctxt->mode;
         int def_op_bytes, def_ad_bytes, group;
 
-        /* Shadow copy of register state. Committed on successful emulation. */
 
+        /* we cannot decode insn before we complete previous rep insn */
+        WARN_ON(ctxt->restart);
+
+        /* Shadow copy of register state. Committed on successful emulation. */
         memset(c, 0, sizeof(struct decode_cache));
         c->eip = ctxt->eip;
         ctxt->cs_base = seg_base(ctxt, VCPU_SREG_CS);
@@ -2426,6 +2429,7 @@ x86_emulate_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
         u64 msr_data;
         struct decode_cache *c = &ctxt->decode;
         int rc = X86EMUL_CONTINUE;
+        int saved_dst_type = c->dst.type;
 
         ctxt->interruptibility = 0;
 
@@ -2454,8 +2458,11 @@ x86_emulate_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
         }
 
         if (c->rep_prefix && (c->d & String)) {
+                ctxt->restart = true;
                 /* All REP prefixes have the same first termination condition */
                 if (address_mask(c, c->regs[VCPU_REGS_RCX]) == 0) {
+                string_done:
+                        ctxt->restart = false;
                         kvm_rip_write(ctxt->vcpu, c->eip);
                         goto done;
                 }
@@ -2467,17 +2474,13 @@ x86_emulate_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
                  *      - if REPNE/REPNZ and ZF = 1 then done
                  */
                 if ((c->b == 0xa6) || (c->b == 0xa7) ||
-                                (c->b == 0xae) || (c->b == 0xaf)) {
+                    (c->b == 0xae) || (c->b == 0xaf)) {
                         if ((c->rep_prefix == REPE_PREFIX) &&
-                                ((ctxt->eflags & EFLG_ZF) == 0)) {
-                                        kvm_rip_write(ctxt->vcpu, c->eip);
-                                        goto done;
-                        }
+                            ((ctxt->eflags & EFLG_ZF) == 0))
+                                goto string_done;
                         if ((c->rep_prefix == REPNE_PREFIX) &&
-                                ((ctxt->eflags & EFLG_ZF) == EFLG_ZF)) {
-                                kvm_rip_write(ctxt->vcpu, c->eip);
-                                goto done;
-                        }
+                            ((ctxt->eflags & EFLG_ZF) == EFLG_ZF))
+                                goto string_done;
                 }
                 c->eip = ctxt->eip;
         }
@@ -2911,6 +2914,12 @@ writeback:
         if (rc != X86EMUL_CONTINUE)
                 goto done;
 
+        /*
+         * restore dst type in case the decoding will be reused
+         * (happens for string instruction )
+         */
+        c->dst.type = saved_dst_type;
+
         if ((c->d & SrcMask) == SrcSI)
                 string_addr_inc(ctxt, seg_override_base(ctxt, c), VCPU_REGS_RSI,
                                 &c->src);
@@ -2918,8 +2927,11 @@ writeback:
         if ((c->d & DstMask) == DstDI)
                 string_addr_inc(ctxt, es_base(ctxt), VCPU_REGS_RDI, &c->dst);
 
-        if (c->rep_prefix && (c->d & String))
+        if (c->rep_prefix && (c->d & String)) {
                 register_address_increment(c, &c->regs[VCPU_REGS_RCX], -1);
+                if (!(c->regs[VCPU_REGS_RCX] & 0x3ff))
+                        ctxt->restart = false;
+        }
 
         /* Commit shadow register state. */
         memcpy(ctxt->vcpu->arch.regs, c->regs, sizeof c->regs);