[PATCH 6/7] 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  |  6 +++
 arch/riscv/kernel/cpufeature.c       |  6 ++-
 arch/riscv/kernel/setup.c            |  1 +
 arch/riscv/kernel/traps_misaligned.c | 63 +++++++++++++++++++++++++++-
 4 files changed, 74 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/include/asm/cpufeature.h b/arch/riscv/include/asm/cpufeature.h
index d0345bd659c9..c1f0ef02cd7d 100644
--- a/arch/riscv/include/asm/cpufeature.h
+++ b/arch/riscv/include/asm/cpufeature.h
@@ -8,6 +8,7 @@
 
 #include <linux/bitmap.h>
 #include <asm/hwcap.h>
+#include <asm/hwprobe.h>
 
 /*
  * These are probed via a device_initcall(), via either the SBI or directly
@@ -32,4 +33,9 @@ extern struct riscv_isainfo hart_isa[NR_CPUS];
 
 void check_unaligned_access(int cpu);
 
+bool unaligned_ctl_available(void);
+
+bool check_unaligned_access_emulated(int cpu);
+void unaligned_emulation_finish(void);
+
 #endif
diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
index 1cfbba65d11a..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");
@@ -645,9 +648,10 @@ void check_unaligned_access(int cpu)
 	__free_pages(page, get_order(MISALIGNED_BUFFER_SIZE));
 }
 
-static int check_unaligned_access_boot_cpu(void)
+static int __init check_unaligned_access_boot_cpu(void)
 {
 	check_unaligned_access(0);
+	unaligned_emulation_finish();
 	return 0;
 }
 
diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
index e600aab116a4..3af6ad4df7cf 100644
--- a/arch/riscv/kernel/setup.c
+++ b/arch/riscv/kernel/setup.c
@@ -26,6 +26,7 @@
 #include <asm/acpi.h>
 #include <asm/alternative.h>
 #include <asm/cacheflush.h>
+#include <asm/cpufeature.h>
 #include <asm/cpu_ops.h>
 #include <asm/early_ioremap.h>
 #include <asm/pgtable.h>
diff --git a/arch/riscv/kernel/traps_misaligned.c b/arch/riscv/kernel/traps_misaligned.c
index b5fb1ff078e3..fa81f6952fa4 100644
--- a/arch/riscv/kernel/traps_misaligned.c
+++ b/arch/riscv/kernel/traps_misaligned.c
@@ -9,11 +9,14 @@
 #include <linux/perf_event.h>
 #include <linux/irq.h>
 #include <linux/stringify.h>
+#include <linux/prctl.h>
 
 #include <asm/processor.h>
 #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
@@ -396,8 +399,10 @@ union reg_data {
 	u64 data_u64;
 };
 
+static bool unaligned_ctl __read_mostly;
+
 /* sysctl hooks */
-int unaligned_enabled __read_mostly = 1;	/* Enabled by default */
+int unaligned_enabled __read_mostly;
 
 int handle_misaligned_load(struct pt_regs *regs)
 {
@@ -412,6 +417,9 @@ int handle_misaligned_load(struct pt_regs *regs)
 	if (!unaligned_enabled)
 		return -1;
 
+	if (user_mode(regs) && (current->thread.align_ctl & PR_UNALIGN_SIGBUS))
+		return -1;
+
 	if (get_insn(regs, epc, &insn))
 		return -1;
 
@@ -511,6 +519,9 @@ int handle_misaligned_store(struct pt_regs *regs)
 	if (!unaligned_enabled)
 		return -1;
 
+	if (user_mode(regs) && (current->thread.align_ctl & PR_UNALIGN_SIGBUS))
+		return -1;
+
 	if (get_insn(regs, epc, &insn))
 		return -1;
 
@@ -585,3 +596,53 @@ int handle_misaligned_store(struct pt_regs *regs)
 
 	return 0;
 }
+
+bool check_unaligned_access_emulated(int cpu)
+{
+	unsigned long emulated = 1, tmp_var;
+
+	/* Use a fixup to detect if misaligned access triggered an exception */
+	__asm__ __volatile__ (
+		"1:\n"
+		"	"REG_L" %[tmp], 1(%[ptr])\n"
+		"	li %[emulated], 0\n"
+		"2:\n"
+		_ASM_EXTABLE(1b, 2b)
+	: [emulated] "+r" (emulated), [tmp] "=r" (tmp_var)
+	: [ptr] "r" (&tmp_var)
+	: "memory");
+
+	if (!emulated)
+		return false;
+
+	per_cpu(misaligned_access_speed, cpu) =
+		RISCV_HWPROBE_MISALIGNED_EMULATED;
+
+	return true;
+}
+
+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_possible_cpu(cpu) {
+		if (per_cpu(misaligned_access_speed, cpu) !=
+		    RISCV_HWPROBE_MISALIGNED_EMULATED) {
+			goto out;
+		}
+	}
+	unaligned_ctl = true;
+
+out:
+	unaligned_enabled = 1;
+}
+
+bool unaligned_ctl_available(void)
+{
+	return unaligned_ctl;
+}
-- 
2.40.1