KVM: x86: fix broken read emulation spans a page boundary

If the range spans a page boundary, the mmio access can be broke, fix it as
write emulation.

And we already get the guest physical address, so use it to read guest data
directly to avoid walking guest page table again
Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/kvm/x86.c

@@ -4045,13 +4045,12 @@ static int vcpu_mmio_gva_to_gpa(struct kvm_vcpu *vcpu, unsigned long gva,
 	return 0;
 }
 
-static int emulator_read_emulated(struct x86_emulate_ctxt *ctxt,
-				  unsigned long addr,
-				  void *val,
-				  unsigned int bytes,
-				  struct x86_exception *exception)
+static int emulator_read_emulated_onepage(unsigned long addr,
+					  void *val,
+					  unsigned int bytes,
+					  struct x86_exception *exception,
+					  struct kvm_vcpu *vcpu)
 {
-	struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
 	gpa_t gpa;
 	int handled, ret;
 
@@ -4071,8 +4070,7 @@ static int emulator_read_emulated(struct x86_emulate_ctxt *ctxt,
 	if (ret)
 		goto mmio;
 
-	if (kvm_read_guest_virt(ctxt, addr, val, bytes, exception)
-	    == X86EMUL_CONTINUE)
+	if (!kvm_read_guest(vcpu->kvm, gpa, val, bytes))
 		return X86EMUL_CONTINUE;
 
 mmio:
@@ -4101,6 +4099,31 @@ static int emulator_read_emulated(struct x86_emulate_ctxt *ctxt,
 	return X86EMUL_IO_NEEDED;
 }
 
+static int emulator_read_emulated(struct x86_emulate_ctxt *ctxt,
+				  unsigned long addr,
+				  void *val,
+				  unsigned int bytes,
+				  struct x86_exception *exception)
+{
+	struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
+
+	/* Crossing a page boundary? */
+	if (((addr + bytes - 1) ^ addr) & PAGE_MASK) {
+		int rc, now;
+
+		now = -addr & ~PAGE_MASK;
+		rc = emulator_read_emulated_onepage(addr, val, now, exception,
+							vcpu);
+		if (rc != X86EMUL_CONTINUE)
+			return rc;
+		addr += now;
+		val += now;
+		bytes -= now;
+	}
+	return emulator_read_emulated_onepage(addr, val, bytes, exception,
+						vcpu);
+}
+
 int emulator_write_phys(struct kvm_vcpu *vcpu, gpa_t gpa,
 			const void *val, int bytes)
 {