[PATCH v3 5/7] RISC-V: hwprobe: Support probing of misaligned access performance

From: Evan Green
Date: Tue Feb 21 2023 - 14:10:03 EST

This allows userspace to select various routines to use based on the
performance of misaligned access on the target hardware.

Co-developed-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
Signed-off-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
Signed-off-by: Evan Green <evan@xxxxxxxxxxxx>

---

Changes in v3:
- Have hwprobe_misaligned return int instead of long.
- Constify cpumask pointer in hwprobe_misaligned()
- Fix warnings in _PERF_O list documentation, use :c:macro:.
- Move include cpufeature.h to misaligned patch.
- Fix documentation mismatch for RISCV_HWPROBE_KEY_CPUPERF_0 (Conor)
- Use for_each_possible_cpu() instead of NR_CPUS (Conor)
- Break early in misaligned access iteration (Conor)
- Increase MISALIGNED_MASK from 2 bits to 3 for possible UNSUPPORTED future
value (Conor)

Changes in v2:
- Fixed logic error in if(of_property_read_string...) that caused crash
- Include cpufeature.h in cpufeature.h to avoid undeclared variable
warning.
- Added a _MASK define
- Fix random checkpatch complaints

Documentation/riscv/hwprobe.rst | 14 ++++++++++++
arch/riscv/include/asm/cpufeature.h | 2 ++
arch/riscv/include/asm/hwprobe.h | 2 +-
arch/riscv/include/asm/smp.h | 11 ++++++++++
arch/riscv/include/uapi/asm/hwprobe.h | 6 ++++++
arch/riscv/kernel/cpufeature.c | 31 +++++++++++++++++++++++++--
arch/riscv/kernel/sys_riscv.c | 27 +++++++++++++++++++++++
7 files changed, 90 insertions(+), 3 deletions(-)

diff --git a/Documentation/riscv/hwprobe.rst b/Documentation/riscv/hwprobe.rst
index 9f2da414fbf8..ac6ded9eb0da 100644
--- a/Documentation/riscv/hwprobe.rst
+++ b/Documentation/riscv/hwprobe.rst
@@ -58,3 +58,17 @@ The following keys are defined:
minimumNumber/maximumNumber, not minNum/maxNum") of the RISC-V ISA manual.
* :c:macro:`RISCV_HWPROBE_IMA_C`: The C extension is supported, as defined
by version 2.2 of the RISC-V ISA manual.
+* :c:macro:`RISCV_HWPROBE_KEY_CPUPERF_0`: A bitmask that contains performance
+ information about the selected set of processors.
+
+ * :c:macro:`RISCV_HWPROBE_MISALIGNED_UNKNOWN`: The performance of misaligned
+ accesses is unknown.
+ * :c:macro:`RISCV_HWPROBE_MISALIGNED_EMULATED`: Misaligned accesses are
+ emulated via software, either in or below the kernel. These accesses are
+ always extremely slow.
+ * :c:macro:`RISCV_HWPROBE_MISALIGNED_SLOW`: Misaligned accesses are supported
+ in hardware, but are slower than the cooresponding aligned accesses
+ sequences.
+ * :c:macro:`RISCV_HWPROBE_MISALIGNED_FAST`: Misaligned accesses are supported
+ in hardware and are faster than the cooresponding aligned accesses
+ sequences.
diff --git a/arch/riscv/include/asm/cpufeature.h b/arch/riscv/include/asm/cpufeature.h
index 66ebaae449c8..808d5403f2ac 100644
--- a/arch/riscv/include/asm/cpufeature.h
+++ b/arch/riscv/include/asm/cpufeature.h
@@ -18,4 +18,6 @@ struct riscv_cpuinfo {

DECLARE_PER_CPU(struct riscv_cpuinfo, riscv_cpuinfo);

+DECLARE_PER_CPU(long, misaligned_access_speed);
+
#endif
diff --git a/arch/riscv/include/asm/hwprobe.h b/arch/riscv/include/asm/hwprobe.h
index 7e52f1e1fe10..4e45e33015bc 100644
--- a/arch/riscv/include/asm/hwprobe.h
+++ b/arch/riscv/include/asm/hwprobe.h
@@ -8,6 +8,6 @@

#include <uapi/asm/hwprobe.h>

-#define RISCV_HWPROBE_MAX_KEY 4
+#define RISCV_HWPROBE_MAX_KEY 5

#endif
diff --git a/arch/riscv/include/asm/smp.h b/arch/riscv/include/asm/smp.h
index 3831b638ecab..4f77a64194e5 100644
--- a/arch/riscv/include/asm/smp.h
+++ b/arch/riscv/include/asm/smp.h
@@ -26,6 +26,17 @@ struct riscv_ipi_ops {
*/
extern unsigned long __cpuid_to_hartid_map[NR_CPUS];
#define cpuid_to_hartid_map(cpu) __cpuid_to_hartid_map[cpu]
+static inline long hartid_to_cpuid_map(unsigned long hartid)
+{
+ long i;
+
+ for_each_possible_cpu(i) {
+ if (cpuid_to_hartid_map(i) == hartid)
+ return i;
+ }
+
+ return -1;
+}

/* print IPI stats */
void show_ipi_stats(struct seq_file *p, int prec);
diff --git a/arch/riscv/include/uapi/asm/hwprobe.h b/arch/riscv/include/uapi/asm/hwprobe.h
index fc5665411782..93fde30fc4a8 100644
--- a/arch/riscv/include/uapi/asm/hwprobe.h
+++ b/arch/riscv/include/uapi/asm/hwprobe.h
@@ -25,6 +25,12 @@ struct riscv_hwprobe {
#define RISCV_HWPROBE_KEY_IMA_EXT_0 4
#define RISCV_HWPROBE_IMA_FD (1 << 0)
#define RISCV_HWPROBE_IMA_C (1 << 1)
+#define RISCV_HWPROBE_KEY_CPUPERF_0 5
+#define RISCV_HWPROBE_MISALIGNED_UNKNOWN (0 << 0)
+#define RISCV_HWPROBE_MISALIGNED_EMULATED (1 << 0)
+#define RISCV_HWPROBE_MISALIGNED_SLOW (2 << 0)
+#define RISCV_HWPROBE_MISALIGNED_FAST (3 << 0)
+#define RISCV_HWPROBE_MISALIGNED_MASK (7 << 0)
/* Increase RISCV_HWPROBE_MAX_KEY when adding items. */

#endif
diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
index 93e45560af30..12af6f7a2f53 100644
--- a/arch/riscv/kernel/cpufeature.c
+++ b/arch/riscv/kernel/cpufeature.c
@@ -14,8 +14,10 @@
#include <linux/of.h>
#include <asm/alternative.h>
#include <asm/cacheflush.h>
+#include <asm/cpufeature.h>
#include <asm/errata_list.h>
#include <asm/hwcap.h>
+#include <asm/hwprobe.h>
#include <asm/patch.h>
#include <asm/pgtable.h>
#include <asm/processor.h>
@@ -32,6 +34,9 @@ static DECLARE_BITMAP(riscv_isa, RISCV_ISA_EXT_MAX) __read_mostly;
DEFINE_STATIC_KEY_ARRAY_FALSE(riscv_isa_ext_keys, RISCV_ISA_EXT_KEY_MAX);
EXPORT_SYMBOL(riscv_isa_ext_keys);

+/* Performance information */
+DEFINE_PER_CPU(long, misaligned_access_speed);
+
/**
* riscv_isa_extension_base() - Get base extension word
*
@@ -89,11 +94,11 @@ static bool riscv_isa_extension_check(int id)
void __init riscv_fill_hwcap(void)
{
struct device_node *node;
- const char *isa;
+ const char *isa, *misaligned;
char print_str[NUM_ALPHA_EXTS + 1];
int i, j, rc;
unsigned long isa2hwcap[26] = {0};
- unsigned long hartid;
+ unsigned long hartid, cpu;

isa2hwcap['i' - 'a'] = COMPAT_HWCAP_ISA_I;
isa2hwcap['m' - 'a'] = COMPAT_HWCAP_ISA_M;
@@ -246,6 +251,28 @@ void __init riscv_fill_hwcap(void)
bitmap_copy(riscv_isa, this_isa, RISCV_ISA_EXT_MAX);
else
bitmap_and(riscv_isa, riscv_isa, this_isa, RISCV_ISA_EXT_MAX);
+
+ /*
+ * Check for the performance of misaligned accesses.
+ */
+ cpu = hartid_to_cpuid_map(hartid);
+ if (cpu < 0)
+ continue;
+
+ if (!of_property_read_string(node, "riscv,misaligned-access-performance",
+ &misaligned)) {
+ if (strcmp(misaligned, "emulated") == 0)
+ per_cpu(misaligned_access_speed, cpu) =
+ RISCV_HWPROBE_MISALIGNED_EMULATED;
+
+ if (strcmp(misaligned, "slow") == 0)
+ per_cpu(misaligned_access_speed, cpu) =
+ RISCV_HWPROBE_MISALIGNED_SLOW;
+
+ if (strcmp(misaligned, "fast") == 0)
+ per_cpu(misaligned_access_speed, cpu) =
+ RISCV_HWPROBE_MISALIGNED_FAST;
+ }
}

/* We don't support systems with F but without D, so mask those out
diff --git a/arch/riscv/kernel/sys_riscv.c b/arch/riscv/kernel/sys_riscv.c
index f2b224550923..afc1b96d5c1a 100644
--- a/arch/riscv/kernel/sys_riscv.c
+++ b/arch/riscv/kernel/sys_riscv.c
@@ -7,6 +7,7 @@

#include <linux/syscalls.h>
#include <asm/cacheflush.h>
+#include <asm/cpufeature.h>
#include <asm/hwprobe.h>
#include <asm/sbi.h>
#include <asm/switch_to.h>
@@ -116,6 +117,28 @@ static void hwprobe_arch_id(struct riscv_hwprobe *pair,
pair->value = id;
}

+static int hwprobe_misaligned(const struct cpumask *cpus)
+{
+ int cpu, perf = -1;
+
+ for_each_cpu(cpu, cpus) {
+ int this_perf = per_cpu(misaligned_access_speed, cpu);
+
+ if (perf == -1)
+ perf = this_perf;
+
+ if (perf != this_perf) {
+ perf = RISCV_HWPROBE_MISALIGNED_UNKNOWN;
+ break;
+ }
+ }
+
+ if (perf == -1)
+ return RISCV_HWPROBE_MISALIGNED_UNKNOWN;
+
+ return perf;
+}
+
static void hwprobe_one_pair(struct riscv_hwprobe *pair,
const struct cpumask *cpus)
{
@@ -145,6 +168,10 @@ static void hwprobe_one_pair(struct riscv_hwprobe *pair,

break;

+ case RISCV_HWPROBE_KEY_CPUPERF_0:
+ pair->value = hwprobe_misaligned(cpus);
+ break;
+
/*
* For forward compatibility, unknown keys don't fail the whole
* call, but get their element key set to -1 and value set to 0
--
2.25.1