[PATCH 8/9] arm64: cpufeature: Expose CPUID registers by emulation

[Suzuki K Poulose <suzuki.poulose@arm.com>]

This patch adds the hook for emulating MRS instruction to
export the 'user visible' value of supported system registers.
We emulate only the following id space for system registers:

 Op0=3, Op1=0, CRn=0, CRm=[0-7]

The rest will fall back to SIGILL. This capability is also
advertised via a new HWCAP_CPUID.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
---
 arch/arm64/include/asm/sysreg.h     |   4 ++
 arch/arm64/include/uapi/asm/hwcap.h |   1 +
 arch/arm64/kernel/cpufeature.c      | 100 ++++++++++++++++++++++++++++++++++++
 arch/arm64/kernel/cpuinfo.c         |   1 +
 4 files changed, 106 insertions(+)

diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
index 488b939..eb1fc46 100644
--- a/arch/arm64/include/asm/sysreg.h
+++ b/arch/arm64/include/asm/sysreg.h
@@ -239,6 +239,10 @@
 #define ID_AA64MMFR0_TGRAN_SUPPORTED	ID_AA64MMFR0_TGRAN64_SUPPORTED
 #endif
 
+
+/* Safe value for MPIDR_EL1: Bit31:RES1, Bit30:U:0, Bit24:MT:1 */
+#define SYS_MPIDR_SAFE_VAL		((1UL<<31)|(1UL<<24))
+
 #ifdef __ASSEMBLY__
 
 	.irp	num,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30
diff --git a/arch/arm64/include/uapi/asm/hwcap.h b/arch/arm64/include/uapi/asm/hwcap.h
index a739287..773c90b 100644
--- a/arch/arm64/include/uapi/asm/hwcap.h
+++ b/arch/arm64/include/uapi/asm/hwcap.h
@@ -30,5 +30,6 @@
 #define HWCAP_ATOMICS		(1 << 8)
 #define HWCAP_FPHP		(1 << 9)
 #define HWCAP_ASIMDHP		(1 << 10)
+#define HWCAP_CPUID		(1 << 11)
 
 #endif /* _UAPI__ASM_HWCAP_H */
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index a7532dd..94c188f 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -29,6 +29,7 @@
 #include <asm/mmu_context.h>
 #include <asm/processor.h>
 #include <asm/sysreg.h>
+#include <asm/traps.h>
 #include <asm/virt.h>
 
 unsigned long elf_hwcap __read_mostly;
@@ -916,6 +917,8 @@ static bool cpus_have_elf_hwcap(const struct arm64_cpu_capabilities *cap)
 
 static void __init setup_elf_hwcaps(const struct arm64_cpu_capabilities *hwcaps)
 {
+	/* We support emulation of accesses to CPU ID feature registers */
+	elf_hwcap |= HWCAP_CPUID;
 	for (; hwcaps->matches; hwcaps++)
 		if (hwcaps->matches(hwcaps, hwcaps->def_scope))
 			cap_set_elf_hwcap(hwcaps);
@@ -1103,3 +1106,100 @@ cpufeature_pan_not_uao(const struct arm64_cpu_capabilities *entry, int __unused)
 {
 	return (cpus_have_cap(ARM64_HAS_PAN) && !cpus_have_cap(ARM64_HAS_UAO));
 }
+
+/*
+ * We emulate only the following system register space.
+ * Op0 = 0x3, CRn = 0x0, Op1 = 0x0, CRm = [0 - 7]
+ * See Table C5-6 System instruction encodings for System register accesses,
+ * ARMv8 ARM(ARM DDI 0487A.f) for more details.
+ */
+static inline bool __attribute_const__ is_emulated(u32 id)
+{
+	return (sys_reg_Op0(id) == 0x3 &&
+		sys_reg_CRn(id) == 0x0 &&
+		sys_reg_Op1(id) == 0x0 &&
+		sys_reg_CRm(id) <= 7);
+}
+
+/*
+ * With CRm == 0, reg should be one of :
+ * MIDR_EL1, MPIDR_EL1 or REVIDR_EL1.
+ */
+static inline int emulate_id_reg(u32 id, u64 *valp)
+{
+	switch(id) {
+	case SYS_MIDR_EL1:
+		*valp = read_cpuid_id();
+		break;
+	case SYS_MPIDR_EL1:
+		*valp = SYS_MPIDR_SAFE_VAL;
+		break;
+	case SYS_REVIDR_EL1:
+		/* IMPLEMENTATION DEFINED values are emulated with 0 */
+		*valp = 0;
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
+static int emulate_sys_reg(u32 id, u64 *valp)
+{
+	struct arm64_ftr_reg *regp;
+
+	if (!is_emulated(id))
+		return -EINVAL;
+
+	if (sys_reg_CRm(id) == 0)
+		return emulate_id_reg(id, valp);
+
+	regp = get_arm64_ftr_reg(id);
+	if (regp)
+		*valp = arm64_ftr_reg_user_value(regp);
+	else
+		/*
+		 * The untracked registers are either IMPLEMENTATION DEFINED
+		 * (e.g, ID_AFR0_EL1) or reserved RAZ.
+		 */
+		*valp = 0;
+	return 0;
+}
+
+static int emulate_mrs(struct pt_regs *regs, u32 insn)
+{
+	int rc;
+	u32 sys_reg, dst;
+	u64 val;
+
+	/*
+	 * sys_reg values are defined as used in mrs/msr instruction.
+	 * shift the imm value to get the encoding.
+	 */
+	sys_reg = (u32)aarch64_insn_decode_immediate(AARCH64_INSN_IMM_16, insn) << 5;
+	rc = emulate_sys_reg(sys_reg, &val);
+	if (!rc) {
+		dst = aarch64_insn_decode_register(AARCH64_INSN_REGTYPE_RT, insn);
+		regs->user_regs.regs[dst] = val;
+		regs->pc += 4;
+	}
+
+	return rc;
+}
+
+static struct undef_hook mrs_hook = {
+	.instr_mask = 0xfff00000,
+	.instr_val  = 0xd5300000,
+	.pstate_mask = COMPAT_PSR_MODE_MASK,
+	.pstate_val = PSR_MODE_EL0t,
+	.fn = emulate_mrs,
+};
+
+int __init enable_mrs_emulation(void)
+{
+	register_undef_hook(&mrs_hook);
+	return 0;
+}
+
+late_initcall(enable_mrs_emulation);
diff --git a/arch/arm64/kernel/cpuinfo.c b/arch/arm64/kernel/cpuinfo.c
index b3d5b3e..ef8406f 100644
--- a/arch/arm64/kernel/cpuinfo.c
+++ b/arch/arm64/kernel/cpuinfo.c
@@ -63,6 +63,7 @@ static const char *const hwcap_str[] = {
 	"atomics",
 	"fphp",
 	"asimdhp",
+	"cpuid",
 	NULL
 };
 
-- 
2.7.4