[PATCH v2 28/30] KVM: x86 emulator: restart string instruction without going back to a guest.

[Gleb Natapov <gleb@redhat.com>]

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 on each page boundary to allow interrupt injection. Pending
exception causes immediate guest entry too.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
---
 arch/x86/include/asm/kvm_emulate.h |    1 +
 arch/x86/kvm/emulate.c             |   26 ++++++++++++++++----------
 arch/x86/kvm/x86.c                 |   19 ++++++++++++++++++-
 3 files changed, 35 insertions(+), 11 deletions(-)

diff --git a/arch/x86/include/asm/kvm_emulate.h b/arch/x86/include/asm/kvm_emulate.h
index 679245c..7fda16f 100644
--- a/arch/x86/include/asm/kvm_emulate.h
+++ b/arch/x86/include/asm/kvm_emulate.h
@@ -193,6 +193,7 @@ struct x86_emulate_ctxt {
 	/* interruptibility state, as a result of execution of STI or MOV SS */
 	int interruptibility;
 
+	bool restart; /* restart string instruction after writeback */
 	/* decode cache */
 	struct decode_cache decode;
 };
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 9d8c7b6..5687b06 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);
@@ -2445,8 +2448,11 @@ x86_emulate_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
 		memop = c->modrm_ea;
 
 	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;
 		}
@@ -2458,17 +2464,13 @@ 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;
 	}
@@ -2904,6 +2906,8 @@ writeback:
 		c->src.ptr = (unsigned long *)
 			register_address(c,  seg_override_base(ctxt, c),
 					 c->regs[VCPU_REGS_RSI]);
+		if (!(c->regs[VCPU_REGS_RSI] & ~PAGE_MASK))
+			ctxt->restart = false;
 	}
 
 	if ((c->d & DstMask) == DstDI) {
@@ -2913,6 +2917,8 @@ writeback:
 		c->dst.ptr = (unsigned long *)
 			register_address(c, es_base(ctxt),
 					 c->regs[VCPU_REGS_RDI]);
+		if (!(c->regs[VCPU_REGS_RDI] & ~PAGE_MASK))
+			ctxt->restart = false;
 	}
 
 	if (c->rep_prefix)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e38ba94..b158ef8 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3702,6 +3702,7 @@ int emulate_instruction(struct kvm_vcpu *vcpu,
 		return EMULATE_DONE;
 	}
 
+restart:
 	r = x86_emulate_insn(&vcpu->arch.emulate_ctxt, &emulate_ops);
 	shadow_mask = vcpu->arch.emulate_ctxt.interruptibility;
 
@@ -3724,7 +3725,7 @@ int emulate_instruction(struct kvm_vcpu *vcpu,
 
 	if (r) {
 		if (kvm_mmu_unprotect_page_virt(vcpu, cr2))
-			return EMULATE_DONE;
+			goto done;
 		if (!vcpu->mmio_needed) {
 			kvm_report_emulation_failure(vcpu, "mmio");
 			return EMULATE_FAIL;
@@ -3739,6 +3740,13 @@ int emulate_instruction(struct kvm_vcpu *vcpu,
 		return EMULATE_DO_MMIO;
 	}
 
+done:
+	if (vcpu->arch.exception.pending)
+		vcpu->arch.emulate_ctxt.restart = false;
+
+	if (vcpu->arch.emulate_ctxt.restart)
+		goto restart;
+
 	return EMULATE_DONE;
 }
 EXPORT_SYMBOL_GPL(emulate_instruction);
@@ -4507,6 +4515,15 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 			goto out;
 		}
 	}
+	if (vcpu->arch.emulate_ctxt.restart) {
+		vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
+		r = emulate_instruction(vcpu, 0, 0, EMULTYPE_NO_DECODE);
+		srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
+		if (r == EMULATE_DO_MMIO) {
+			r = 0;
+			goto out;
+		}
+	}
 	if (kvm_run->exit_reason == KVM_EXIT_HYPERCALL)
 		kvm_register_write(vcpu, VCPU_REGS_RAX,
 				     kvm_run->hypercall.ret);
-- 
1.6.5