KVM: MIPS: Implement HYPCALL emulation

Emulate the HYPCALL instruction added in the VZ ASE and used by the MIPS
paravirtualised guest support that is already merged. The new hypcall.c
handles arguments and the return value. No actual hypercalls are yet
supported, but this still allows us to safely step over hypercalls and
set an error code in the return value for forward compatibility.

Non-zero HYPCALL codes are not handled.

We also document the hypercall ABI which asm/kvm_para.h uses.

Signed-off-by: James Hogan <[e-mail]>
Acked-by: Ralf Baechle <[e-mail]>
Cc: Paolo Bonzini <[e-mail]>
Cc: "Radim Krčmář" <[e-mail]>
Cc: Andreas Herrmann <[e-mail]>
Cc: David Daney <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]

Documentation/virtual/kvm/hypercalls.txt

···
   property inside the device tree's /hypervisor node.
   For more information refer to Documentation/virtual/kvm/ppc-pv.txt
 
+MIPS:
+  KVM hypercalls use the HYPCALL instruction with code 0 and the hypercall
+  number in $2 (v0). Up to four arguments may be placed in $4-$7 (a0-a3) and
+  the return value is placed in $2 (v0).
+
 KVM Hypercalls Documentation
 ===========================
 The template for each hypercall is:

arch/mips/include/asm/kvm_host.h

···
 	EMULATE_WAIT,		/* WAIT instruction */
 	EMULATE_PRIV_FAIL,
 	EMULATE_EXCEPT,		/* A guest exception has been generated */
+	EMULATE_HYPERCALL,	/* HYPCALL instruction */
 };
 
 #define mips3_paddr_to_tlbpfn(x) \
···
 unsigned int kvm_mips_config3_wrmask(struct kvm_vcpu *vcpu);
 unsigned int kvm_mips_config4_wrmask(struct kvm_vcpu *vcpu);
 unsigned int kvm_mips_config5_wrmask(struct kvm_vcpu *vcpu);
+
+/* Hypercalls (hypcall.c) */
+
+enum emulation_result kvm_mips_emul_hypcall(struct kvm_vcpu *vcpu,
+					    union mips_instruction inst);
+int kvm_mips_handle_hypcall(struct kvm_vcpu *vcpu);
 
 /* Dynamic binary translation */
 extern int kvm_mips_trans_cache_index(union mips_instruction inst,

arch/mips/include/uapi/asm/inst.h

···
 	tlbr_op	      = 0x01, tlbwi_op	    = 0x02,
 	tlbwr_op      = 0x06, tlbp_op	    = 0x08,
 	rfe_op	      = 0x10, eret_op	    = 0x18,
-	wait_op       = 0x20,
+	wait_op       = 0x20, hypcall_op    = 0x28
 };
 
 /*

arch/mips/kvm/Makefile

···
 kvm-objs := $(common-objs-y) mips.o emulate.o entry.o \
 	    interrupt.o stats.o commpage.o \
 	    dyntrans.o trap_emul.o fpu.o
+kvm-objs += hypcall.o
 kvm-objs += mmu.o
 
 obj-$(CONFIG_KVM)	+= kvm.o

arch/mips/kvm/emulate.c

···
 		case wait_op:
 			er = kvm_mips_emul_wait(vcpu);
 			break;
+		case hypcall_op:
+			er = kvm_mips_emul_hypcall(vcpu, inst);
+			break;
 		}
 	} else {
 		rt = inst.c0r_format.rt;

arch/mips/kvm/hypcall.c

···
+/*
+ * This file is subject to the terms and conditions of the GNU General Public
+ * License.  See the file "COPYING" in the main directory of this archive
+ * for more details.
+ *
+ * KVM/MIPS: Hypercall handling.
+ *
+ * Copyright (C) 2015  Imagination Technologies Ltd.
+ */
+
+#include <linux/kernel.h>
+#include <linux/kvm_host.h>
+#include <linux/kvm_para.h>
+
+#define MAX_HYPCALL_ARGS	4
+
+enum emulation_result kvm_mips_emul_hypcall(struct kvm_vcpu *vcpu,
+					    union mips_instruction inst)
+{
+	unsigned int code = (inst.co_format.code >> 5) & 0x3ff;
+
+	kvm_debug("[%#lx] HYPCALL %#03x\n", vcpu->arch.pc, code);
+
+	switch (code) {
+	case 0:
+		return EMULATE_HYPERCALL;
+	default:
+		return EMULATE_FAIL;
+	};
+}
+
+static int kvm_mips_hypercall(struct kvm_vcpu *vcpu, unsigned long num,
+			      const unsigned long *args, unsigned long *hret)
+{
+	/* Report unimplemented hypercall to guest */
+	*hret = -KVM_ENOSYS;
+	return RESUME_GUEST;
+}
+
+int kvm_mips_handle_hypcall(struct kvm_vcpu *vcpu)
+{
+	unsigned long num, args[MAX_HYPCALL_ARGS];
+
+	/* read hypcall number and arguments */
+	num = vcpu->arch.gprs[2];	/* v0 */
+	args[0] = vcpu->arch.gprs[4];	/* a0 */
+	args[1] = vcpu->arch.gprs[5];	/* a1 */
+	args[2] = vcpu->arch.gprs[6];	/* a2 */
+	args[3] = vcpu->arch.gprs[7];	/* a3 */
+
+	return kvm_mips_hypercall(vcpu, num,
+				  args, &vcpu->arch.gprs[2] /* v0 */);
+}

arch/mips/kvm/trap_emul.c

···
 		ret = RESUME_HOST;
 		break;
 
+	case EMULATE_HYPERCALL:
+		ret = kvm_mips_handle_hypcall(vcpu);
+		break;
+
 	default:
 		BUG();
 	}