[PATCH v3 04/17] riscv: misaligned: request misaligned exception from SBI

Clément Léger
2025-03-10T15:13:20Z

Now that the kernel can handle misaligned accesses in S-mode, request
misaligned access exception delegation from SBI. This uses the FWFT SBI
extension defined in SBI version 3.0.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
---
arch/riscv/include/asm/cpufeature.h | 3 +-
arch/riscv/kernel/traps_misaligned.c | 77 +++++++++++++++++++++-
arch/riscv/kernel/unaligned_access_speed.c | 11 +++-
3 files changed, 86 insertions(+), 5 deletions(-)

diff --git a/arch/riscv/include/asm/cpufeature.h b/arch/riscv/include/asm/cpufeature.h
index 569140d6e639..ad7d26788e6a 100644
--- a/arch/riscv/include/asm/cpufeature.h
+++ b/arch/riscv/include/asm/cpufeature.h
@@ -64,8 +64,9 @@ void __init riscv_user_isa_enable(void);
_RISCV_ISA_EXT_DATA(_name, _id, _sub_exts, ARRAY_SIZE(_sub_exts), _validate)

bool check_unaligned_access_emulated_all_cpus(void);
+void unaligned_access_init(void);
+int cpu_online_unaligned_access_init(unsigned int cpu);
#if defined(CONFIG_RISCV_SCALAR_MISALIGNED)
-void check_unaligned_access_emulated(struct work_struct *work __always_unused);
void unaligned_emulation_finish(void);
bool unaligned_ctl_available(void);
DECLARE_PER_CPU(long, misaligned_access_speed);
diff --git a/arch/riscv/kernel/traps_misaligned.c b/arch/riscv/kernel/traps_misaligned.c
index 7cc108aed74e..90ac74191357 100644
--- a/arch/riscv/kernel/traps_misaligned.c
+++ b/arch/riscv/kernel/traps_misaligned.c
@@ -16,6 +16,7 @@
#include <asm/entry-common.h>
#include <asm/hwprobe.h>
#include <asm/cpufeature.h>
+#include <asm/sbi.h>
#include <asm/vector.h>

#define INSN_MATCH_LB 0x3
@@ -635,7 +636,7 @@ bool check_vector_unaligned_access_emulated_all_cpus(void)

static bool unaligned_ctl __read_mostly;

-void check_unaligned_access_emulated(struct work_struct *work __always_unused)
+static void check_unaligned_access_emulated(struct work_struct *work __always_unused)
{
int cpu = smp_processor_id();
long *mas_ptr = per_cpu_ptr(&misaligned_access_speed, cpu);
@@ -646,6 +647,13 @@ void check_unaligned_access_emulated(struct work_struct *work __always_unused)
__asm__ __volatile__ (
" "REG_L" %[tmp], 1(%[ptr])\n"
: [tmp] "=r" (tmp_val) : [ptr] "r" (&tmp_var) : "memory");
+}
+
+static int cpu_online_check_unaligned_access_emulated(unsigned int cpu)
+{
+ long *mas_ptr = per_cpu_ptr(&misaligned_access_speed, cpu);
+
+ check_unaligned_access_emulated(NULL);

/*
* If unaligned_ctl is already set, this means that we detected that all
@@ -654,9 +662,10 @@ void check_unaligned_access_emulated(struct work_struct *work __always_unused)
*/
if (unlikely(unaligned_ctl && (*mas_ptr != RISCV_HWPROBE_MISALIGNED_SCALAR_EMULATED))) {
pr_crit("CPU misaligned accesses non homogeneous (expected all emulated)\n");
- while (true)
- cpu_relax();
+ return -EINVAL;
}
+
+ return 0;
}

bool check_unaligned_access_emulated_all_cpus(void)
@@ -688,4 +697,66 @@ bool check_unaligned_access_emulated_all_cpus(void)
{
return false;
}
+static int cpu_online_check_unaligned_access_emulated(unsigned int cpu)
+{
+ return 0;
+}
#endif
+
+#ifdef CONFIG_RISCV_SBI
+
+static bool misaligned_traps_delegated;
+
+static int cpu_online_sbi_unaligned_setup(unsigned int cpu)
+{
+ if (sbi_fwft_set(SBI_FWFT_MISALIGNED_EXC_DELEG, 1, 0) &&
+ misaligned_traps_delegated) {
+ pr_crit("Misaligned trap delegation non homogeneous (expected delegated)");
+ return -EINVAL;
+ }
+
+ return 0;
+}
+
+static void unaligned_sbi_request_delegation(void)
+{
+ int ret;
+
+ ret = sbi_fwft_all_cpus_set(SBI_FWFT_MISALIGNED_EXC_DELEG, 1, 0, 0);
+ if (ret)
+ return;
+
+ misaligned_traps_delegated = true;
+ pr_info("SBI misaligned access exception delegation ok\n");
+ /*
+ * Note that we don't have to take any specific action here, if
+ * the delegation is successful, then
+ * check_unaligned_access_emulated() will verify that indeed the
+ * platform traps on misaligned accesses.
+ */
+}
+
+void unaligned_access_init(void)
+{
+ if (sbi_probe_extension(SBI_EXT_FWFT) > 0)
+ unaligned_sbi_request_delegation();
+}
+#else
+void unaligned_access_init(void) {}
+
+static int cpu_online_sbi_unaligned_setup(unsigned int cpu __always_unused)
+{
+ return 0;
+}
+#endif
+
+int cpu_online_unaligned_access_init(unsigned int cpu)
+{
+ int ret;
+
+ ret = cpu_online_sbi_unaligned_setup(cpu);
+ if (ret)
+ return ret;
+
+ return cpu_online_check_unaligned_access_emulated(cpu);
+}
diff --git a/arch/riscv/kernel/unaligned_access_speed.c b/arch/riscv/kernel/unaligned_access_speed.c
index 91f189cf1611..2f3aba073297 100644
--- a/arch/riscv/kernel/unaligned_access_speed.c
+++ b/arch/riscv/kernel/unaligned_access_speed.c
@@ -188,13 +188,20 @@ arch_initcall_sync(lock_and_set_unaligned_access_static_branch);

static int riscv_online_cpu(unsigned int cpu)
{
+ int ret;
static struct page *buf;

/* We are already set since the last check */
if (per_cpu(misaligned_access_speed, cpu) != RISCV_HWPROBE_MISALIGNED_SCALAR_UNKNOWN)
goto exit;

- check_unaligned_access_emulated(NULL);
+ ret = cpu_online_unaligned_access_init(cpu);
+ if (ret)
+ return ret;
+
+ if (per_cpu(misaligned_access_speed, cpu) == RISCV_HWPROBE_MISALIGNED_SCALAR_EMULATED)
+ goto exit;
+
buf = alloc_pages(GFP_KERNEL, MISALIGNED_BUFFER_ORDER);
if (!buf) {
pr_warn("Allocation failure, not measuring misaligned performance\n");
@@ -403,6 +410,8 @@ static int check_unaligned_access_all_cpus(void)
{
bool all_cpus_emulated, all_cpus_vec_unsupported;

+ unaligned_access_init();
+
all_cpus_emulated = check_unaligned_access_emulated_all_cpus();
all_cpus_vec_unsupported = check_vector_unaligned_access_emulated_all_cpus();

--
2.47.2