[PATCH v2 7/8] riscv: report misaligned accesses emulation to hwprobe

[Clément Léger]

hwprobe provides a way to report if misaligned access are emulated. In
order to correctly populate that feature, we can check if it actually
traps when doing a misaligned access. This can be checked using an
exception table entry which will actually be used when a misaligned
access is done from kernel mode.

Signed-off-by: Clément Léger <cleger at rivosinc.com>
---
 arch/riscv/include/asm/cpufeature.h  | 18 +++++++++
 arch/riscv/kernel/cpufeature.c       |  4 ++
 arch/riscv/kernel/smpboot.c          |  2 +-
 arch/riscv/kernel/traps_misaligned.c | 56 ++++++++++++++++++++++++++++
 4 files changed, 79 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/include/asm/cpufeature.h b/arch/riscv/include/asm/cpufeature.h
index d0345bd659c9..e4ae6af51876 100644
--- a/arch/riscv/include/asm/cpufeature.h
+++ b/arch/riscv/include/asm/cpufeature.h
@@ -32,4 +32,22 @@ extern struct riscv_isainfo hart_isa[NR_CPUS];
 
 void check_unaligned_access(int cpu);
 
+#ifdef CONFIG_RISCV_MISALIGNED
+bool unaligned_ctl_available(void);
+bool check_unaligned_access_emulated(int cpu);
+void unaligned_emulation_finish(void);
+#else
+static inline bool unaligned_ctl_available(void)
+{
+	return false;
+}
+
+static inline bool check_unaligned_access_emulated(int cpu)
+{
+	return false;
+}
+
+static inline void unaligned_emulation_finish(void) {}
+#endif
+
 #endif
diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
index 356e5677eeb1..fbbde800bc21 100644
--- a/arch/riscv/kernel/cpufeature.c
+++ b/arch/riscv/kernel/cpufeature.c
@@ -568,6 +568,9 @@ void check_unaligned_access(int cpu)
 	void *src;
 	long speed = RISCV_HWPROBE_MISALIGNED_SLOW;
 
+	if (check_unaligned_access_emulated(cpu))
+		return;
+
 	page = alloc_pages(GFP_NOWAIT, get_order(MISALIGNED_BUFFER_SIZE));
 	if (!page) {
 		pr_warn("Can't alloc pages to measure memcpy performance");
@@ -648,6 +651,7 @@ void check_unaligned_access(int cpu)
 static int __init check_unaligned_access_boot_cpu(void)
 {
 	check_unaligned_access(0);
+	unaligned_emulation_finish();
 	return 0;
 }
 
diff --git a/arch/riscv/kernel/smpboot.c b/arch/riscv/kernel/smpboot.c
index 1b8da4e40a4d..5d9858d6ad26 100644
--- a/arch/riscv/kernel/smpboot.c
+++ b/arch/riscv/kernel/smpboot.c
@@ -245,8 +245,8 @@ asmlinkage __visible void smp_callin(void)
 	riscv_ipi_enable();
 
 	numa_add_cpu(curr_cpuid);
-	set_cpu_online(curr_cpuid, 1);
 	check_unaligned_access(curr_cpuid);
+	set_cpu_online(curr_cpuid, 1);
 
 	if (has_vector()) {
 		if (riscv_v_setup_vsize())
diff --git a/arch/riscv/kernel/traps_misaligned.c b/arch/riscv/kernel/traps_misaligned.c
index b5fb1ff078e3..d99b95084b6c 100644
--- a/arch/riscv/kernel/traps_misaligned.c
+++ b/arch/riscv/kernel/traps_misaligned.c
@@ -14,6 +14,8 @@
 #include <asm/ptrace.h>
 #include <asm/csr.h>
 #include <asm/entry-common.h>
+#include <asm/hwprobe.h>
+#include <asm/cpufeature.h>
 
 #define INSN_MATCH_LB			0x3
 #define INSN_MASK_LB			0x707f
@@ -396,6 +398,8 @@ union reg_data {
 	u64 data_u64;
 };
 
+static bool unaligned_ctl __read_mostly;
+
 /* sysctl hooks */
 int unaligned_enabled __read_mostly = 1;	/* Enabled by default */
 
@@ -409,6 +413,8 @@ int handle_misaligned_load(struct pt_regs *regs)
 
 	perf_sw_event(PERF_COUNT_SW_ALIGNMENT_FAULTS, 1, regs, addr);
 
+	*this_cpu_ptr(&misaligned_access_speed) = RISCV_HWPROBE_MISALIGNED_EMULATED;
+
 	if (!unaligned_enabled)
 		return -1;
 
@@ -585,3 +591,53 @@ int handle_misaligned_store(struct pt_regs *regs)
 
 	return 0;
 }
+
+bool check_unaligned_access_emulated(int cpu)
+{
+	long *mas_ptr = per_cpu_ptr(&misaligned_access_speed, cpu);
+	unsigned long tmp_var, tmp_val;
+	bool misaligned_emu_detected;
+
+	*mas_ptr = RISCV_HWPROBE_MISALIGNED_UNKNOWN;
+
+	__asm__ __volatile__ (
+		"       "REG_L" %[tmp], 1(%[ptr])\n"
+		: [tmp] "=r" (tmp_val) : [ptr] "r" (&tmp_var) : "memory");
+
+	misaligned_emu_detected = (*mas_ptr == RISCV_HWPROBE_MISALIGNED_EMULATED);
+	/*
+	 * If unaligned_ctl is already set, this means that we detected that all
+	 * CPUS uses emulated misaligned access at boot time. If that changed
+	 * when hotplugging the new cpu, this is something we don't handle.
+	 */
+	if (unlikely(unaligned_ctl && !misaligned_emu_detected)) {
+		pr_crit("CPU misaligned accesses non homogeneous (expected all emulated)\n");
+		while (true)
+			cpu_relax();
+	}
+
+	return misaligned_emu_detected;
+}
+
+void __init unaligned_emulation_finish(void)
+{
+	int cpu;
+
+	/*
+	 * We can only support PR_UNALIGN controls if all CPUs have misaligned
+	 * accesses emulated since tasks requesting such control can run on any
+	 * CPU.
+	 */
+	for_each_present_cpu(cpu) {
+		if (per_cpu(misaligned_access_speed, cpu) !=
+					RISCV_HWPROBE_MISALIGNED_EMULATED) {
+			return;
+		}
+	}
+	unaligned_ctl = true;
+}
+
+bool unaligned_ctl_available(void)
+{
+	return unaligned_ctl;
+}
-- 
2.42.0