Re: [PATCH v6 6/8] RISC-V: Detect unaligned vector accesses supported

From: Jesse Taube
Date: Thu Jul 25 2024 - 15:58:52 EST

On Thu, Jul 25, 2024 at 2:59 PM Evan Green <evan@xxxxxxxxxxxx> wrote:
>
> On Wed, Jul 24, 2024 at 11:36 AM Jesse Taube <jesse@xxxxxxxxxxxx> wrote:
> >
> > Run a unaligned vector access to test if the system supports
> > vector unaligned access. Add the result to a new key in hwprobe.
> > This is useful for usermode to know if vector misaligned accesses are
> > supported and if they are faster or slower than equivalent byte accesses.
> >
> > Signed-off-by: Jesse Taube <jesse@xxxxxxxxxxxx>
> > Reviewed-by: Charlie Jenkins <charlie@xxxxxxxxxxxx>
> > ---
> > V1 -> V2:
> > - Add Kconfig options
> > - Add insn_is_vector
> > - Add handle_vector_misaligned_load
> > - Fix build
> > - Seperate vector from scalar misaligned access
> > - This patch was almost completely rewritten
> > V2 -> V3:
> > - Fixed CONFIG_ in Kconfig
> > - Fixed check_vector_unaligned_access_emulated leaving
> > vector_misaligned_access as unknown.
> > - Remove local_irq_enable
> > - Remove RISCV_DETECT_VECTOR_UNALIGNED_ACCESS
> > - Remove RISCV_VEC_UNALIGNED_ACCESS_UNSUPPORTED
> > V3 -> V4:
> > - Spell out _VECTOR_ in macros
> > V4 -> V5:
> > - Change work_struct *unused to work_struct *work __always_unused
> > - Add insn_is_vector definition to vector.h when V is not defined
> > V5 -> V6:
> > - Change check_vector_unaligned_access_emulated to extern
> > - Move check_unaligned_access_emulated_all_cpus out of the #ifdef
> > see last commit
> > ---
> > arch/riscv/Kconfig | 35 ++++++
> > arch/riscv/include/asm/cpufeature.h | 8 +-
> > arch/riscv/include/asm/entry-common.h | 11 --
> > arch/riscv/include/asm/hwprobe.h | 2 +-
> > arch/riscv/include/asm/vector.h | 2 +
> > arch/riscv/include/uapi/asm/hwprobe.h | 5 +
> > arch/riscv/kernel/Makefile | 4 +-
> > arch/riscv/kernel/sys_hwprobe.c | 35 ++++++
> > arch/riscv/kernel/traps_misaligned.c | 120 ++++++++++++++++++++-
> > arch/riscv/kernel/unaligned_access_speed.c | 22 ++--
> > arch/riscv/kernel/vector.c | 2 +-
> > 11 files changed, 216 insertions(+), 30 deletions(-)
> >
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index 34d24242e37a..ffbe0fdd7fb3 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -717,12 +717,26 @@ config THREAD_SIZE_ORDER
> > Specify the Pages of thread stack size (from 4KB to 64KB), which also
> > affects irq stack size, which is equal to thread stack size.
> >
> > +config RISCV_MISALIGNED
> > + bool
> > + help
> > + Embed support for detecting and emulating misaligned
> > + scalar or vector loads and stores.
> > +
> > config RISCV_SCALAR_MISALIGNED
> > bool
> > + select RISCV_MISALIGNED
> > select SYSCTL_ARCH_UNALIGN_ALLOW
> > help
> > Embed support for emulating misaligned loads and stores.
> >
> > +config RISCV_VECTOR_MISALIGNED
> > + bool
> > + select RISCV_MISALIGNED
> > + depends on RISCV_ISA_V
> > + help
> > + Enable detecting support for vector misaligned loads and stores.
> > +
> > choice
> > prompt "Unaligned Accesses Support"
> > default RISCV_PROBE_UNALIGNED_ACCESS
> > @@ -774,6 +788,27 @@ config RISCV_EFFICIENT_UNALIGNED_ACCESS
> >
> > endchoice
> >
> > +choice
> > + prompt "Vector unaligned Accesses Support"
> > + depends on RISCV_ISA_V
> > + default RISCV_PROBE_VECTOR_UNALIGNED_ACCESS
> > + help
> > + This determines the level of support for vector unaligned accesses. This
> > + information is used by the kernel to perform optimizations. It is also
> > + exposed to user space via the hwprobe syscall. The hardware will be
> > + probed at boot by default.
> > +
> > +config RISCV_PROBE_VECTOR_UNALIGNED_ACCESS
> > + bool "Probe speed of vector unaligned accesses"
> > + select RISCV_VECTOR_MISALIGNED
> > + help
> > + During boot, the kernel will run a series of tests to determine the
> > + speed of vector unaligned accesses if they are supported. This probing
> > + will dynamically determine the speed of vector unaligned accesses on
> > + the underlying system if they are supported.
> > +
> > +endchoice
> > +
> > endmenu # "Platform type"
> >
> > menu "Kernel features"
> > diff --git a/arch/riscv/include/asm/cpufeature.h b/arch/riscv/include/asm/cpufeature.h
> > index 4ade9f87fc55..5634f702f3fe 100644
> > --- a/arch/riscv/include/asm/cpufeature.h
> > +++ b/arch/riscv/include/asm/cpufeature.h
> > @@ -34,8 +34,8 @@ extern struct riscv_isainfo hart_isa[NR_CPUS];
> >
> > void riscv_user_isa_enable(void);
> >
> > -#if defined(CONFIG_RISCV_SCALAR_MISALIGNED)
> > bool check_unaligned_access_emulated_all_cpus(void);
> > +#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);
> > @@ -47,6 +47,12 @@ static inline bool unaligned_ctl_available(void)
> > }
> > #endif
> >
> > +bool check_vector_unaligned_access_emulated_all_cpus(void);
> > +#if defined(CONFIG_RISCV_VECTOR_MISALIGNED)
> > +void check_vector_unaligned_access_emulated(struct work_struct *work __always_unused);
> > +DECLARE_PER_CPU(long, vector_misaligned_access);
> > +#endif
> > +
> > #if defined(CONFIG_RISCV_PROBE_UNALIGNED_ACCESS)
> > DECLARE_STATIC_KEY_FALSE(fast_unaligned_access_speed_key);
> >
> > diff --git a/arch/riscv/include/asm/entry-common.h b/arch/riscv/include/asm/entry-common.h
> > index 0a4e3544c877..7b32d2b08bb6 100644
> > --- a/arch/riscv/include/asm/entry-common.h
> > +++ b/arch/riscv/include/asm/entry-common.h
> > @@ -25,18 +25,7 @@ static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
> > void handle_page_fault(struct pt_regs *regs);
> > void handle_break(struct pt_regs *regs);
> >
> > -#ifdef CONFIG_RISCV_SCALAR_MISALIGNED
> > int handle_misaligned_load(struct pt_regs *regs);
> > int handle_misaligned_store(struct pt_regs *regs);
> > -#else
> > -static inline int handle_misaligned_load(struct pt_regs *regs)
> > -{
> > - return -1;
> > -}
> > -static inline int handle_misaligned_store(struct pt_regs *regs)
> > -{
> > - return -1;
> > -}
> > -#endif
> >
> > #endif /* _ASM_RISCV_ENTRY_COMMON_H */
> > diff --git a/arch/riscv/include/asm/hwprobe.h b/arch/riscv/include/asm/hwprobe.h
> > index 150a9877b0af..ef01c182af2b 100644
> > --- a/arch/riscv/include/asm/hwprobe.h
> > +++ b/arch/riscv/include/asm/hwprobe.h
> > @@ -8,7 +8,7 @@
> >
> > #include <uapi/asm/hwprobe.h>
> >
> > -#define RISCV_HWPROBE_MAX_KEY 7
> > +#define RISCV_HWPROBE_MAX_KEY 8
> >
> > static inline bool riscv_hwprobe_key_is_valid(__s64 key)
> > {
> > diff --git a/arch/riscv/include/asm/vector.h b/arch/riscv/include/asm/vector.h
> > index be7d309cca8a..c7c023afbacd 100644
> > --- a/arch/riscv/include/asm/vector.h
> > +++ b/arch/riscv/include/asm/vector.h
> > @@ -21,6 +21,7 @@
> >
> > extern unsigned long riscv_v_vsize;
> > int riscv_v_setup_vsize(void);
> > +bool insn_is_vector(u32 insn_buf);
> > bool riscv_v_first_use_handler(struct pt_regs *regs);
> > void kernel_vector_begin(void);
> > void kernel_vector_end(void);
> > @@ -268,6 +269,7 @@ struct pt_regs;
> >
> > static inline int riscv_v_setup_vsize(void) { return -EOPNOTSUPP; }
> > static __always_inline bool has_vector(void) { return false; }
> > +static __always_inline bool insn_is_vector(u32 insn_buf) { return false; }
> > static inline bool riscv_v_first_use_handler(struct pt_regs *regs) { return false; }
> > static inline bool riscv_v_vstate_query(struct pt_regs *regs) { return false; }
> > static inline bool riscv_v_vstate_ctrl_user_allowed(void) { return false; }
> > diff --git a/arch/riscv/include/uapi/asm/hwprobe.h b/arch/riscv/include/uapi/asm/hwprobe.h
> > index 023b7771d1b7..48b92fb07edf 100644
> > --- a/arch/riscv/include/uapi/asm/hwprobe.h
> > +++ b/arch/riscv/include/uapi/asm/hwprobe.h
> > @@ -75,6 +75,11 @@ struct riscv_hwprobe {
> > #define RISCV_HWPROBE_MISALIGNED_MASK (7 << 0)
> > #define RISCV_HWPROBE_KEY_ZICBOZ_BLOCK_SIZE 6
> > #define RISCV_HWPROBE_KEY_MISALIGNED_PERF 7
> > +#define RISCV_HWPROBE_KEY_VECTOR_MISALIGNED_PERF 8
> > +#define RISCV_HWPROBE_VECTOR_MISALIGNED_UNKNOWN 0
> > +#define RISCV_HWPROBE_VECTOR_MISALIGNED_SLOW 2
> > +#define RISCV_HWPROBE_VECTOR_MISALIGNED_FAST 3
> > +#define RISCV_HWPROBE_VECTOR_MISALIGNED_UNSUPPORTED 4
> > /* Increase RISCV_HWPROBE_MAX_KEY when adding items. */
> >
> > /* Flags */
> > diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
> > index 8d4e7d40e42f..5b243d46f4b1 100644
> > --- a/arch/riscv/kernel/Makefile
> > +++ b/arch/riscv/kernel/Makefile
> > @@ -62,8 +62,8 @@ obj-y += probes/
> > obj-y += tests/
> > obj-$(CONFIG_MMU) += vdso.o vdso/
> >
> > -obj-$(CONFIG_RISCV_SCALAR_MISALIGNED) += traps_misaligned.o
> > -obj-$(CONFIG_RISCV_SCALAR_MISALIGNED) += unaligned_access_speed.o
> > +obj-$(CONFIG_RISCV_MISALIGNED) += traps_misaligned.o
> > +obj-$(CONFIG_RISCV_MISALIGNED) += unaligned_access_speed.o
> > obj-$(CONFIG_RISCV_PROBE_UNALIGNED_ACCESS) += copy-unaligned.o
> >
> > obj-$(CONFIG_FPU) += fpu.o
> > diff --git a/arch/riscv/kernel/sys_hwprobe.c b/arch/riscv/kernel/sys_hwprobe.c
> > index e910e2971984..2e3e324bad38 100644
> > --- a/arch/riscv/kernel/sys_hwprobe.c
> > +++ b/arch/riscv/kernel/sys_hwprobe.c
> > @@ -194,6 +194,37 @@ static u64 hwprobe_misaligned(const struct cpumask *cpus)
> > }
> > #endif
> >
> > +#ifdef CONFIG_RISCV_VECTOR_MISALIGNED
> > +static u64 hwprobe_vec_misaligned(const struct cpumask *cpus)
> > +{
> > + int cpu;
> > + u64 perf = -1ULL;
> > +
> > + /* Return if supported or not even if speed wasn't probed */
> > + for_each_cpu(cpu, cpus) {
> > + int this_perf = per_cpu(vector_misaligned_access, cpu);
> > +
> > + if (perf == -1ULL)
> > + perf = this_perf;
> > +
> > + if (perf != this_perf) {
> > + perf = RISCV_HWPROBE_VECTOR_MISALIGNED_UNKNOWN;
> > + break;
> > + }
> > + }
> > +
> > + if (perf == -1ULL)
> > + return RISCV_HWPROBE_VECTOR_MISALIGNED_UNKNOWN;
> > +
> > + return perf;
> > +}
> > +#else
> > +static u64 hwprobe_vec_misaligned(const struct cpumask *cpus)
> > +{
> > + return RISCV_HWPROBE_VECTOR_MISALIGNED_UNKNOWN;
> > +}
> > +#endif
> > +
> > static void hwprobe_one_pair(struct riscv_hwprobe *pair,
> > const struct cpumask *cpus)
> > {
> > @@ -222,6 +253,10 @@ static void hwprobe_one_pair(struct riscv_hwprobe *pair,
> > pair->value = hwprobe_misaligned(cpus);
> > break;
> >
> > + case RISCV_HWPROBE_KEY_VECTOR_MISALIGNED_PERF:
> > + pair->value = hwprobe_vec_misaligned(cpus);
> > + break;
> > +
> > case RISCV_HWPROBE_KEY_ZICBOZ_BLOCK_SIZE:
> > pair->value = 0;
> > if (hwprobe_ext0_has(cpus, RISCV_HWPROBE_EXT_ZICBOZ))
> > diff --git a/arch/riscv/kernel/traps_misaligned.c b/arch/riscv/kernel/traps_misaligned.c
> > index bb09357778c5..817496a6a2d5 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/vector.h>
> >
> > #define INSN_MATCH_LB 0x3
> > #define INSN_MASK_LB 0x707f
> > @@ -322,12 +323,37 @@ union reg_data {
> > u64 data_u64;
> > };
> >
> > -static bool unaligned_ctl __read_mostly;
> > -
> > /* sysctl hooks */
> > int unaligned_enabled __read_mostly = 1; /* Enabled by default */
> >
> > -int handle_misaligned_load(struct pt_regs *regs)
> > +#ifdef CONFIG_RISCV_VECTOR_MISALIGNED
> > +static int handle_vector_misaligned_load(struct pt_regs *regs)
> > +{
> > + unsigned long epc = regs->epc;
> > + unsigned long insn;
> > +
> > + if (get_insn(regs, epc, &insn))
> > + return -1;
> > +
> > + /* Only return 0 when in check_vector_unaligned_access_emulated */
> > + if (*this_cpu_ptr(&vector_misaligned_access) == RISCV_HWPROBE_VECTOR_MISALIGNED_UNKNOWN) {
> > + *this_cpu_ptr(&vector_misaligned_access) = RISCV_HWPROBE_VECTOR_MISALIGNED_UNSUPPORTED;
> > + regs->epc = epc + INSN_LEN(insn);
> > + return 0;
> > + }
> > +
> > + /* If vector instruction we don't emulate it yet */
> > + regs->epc = epc;
> > + return -1;
> > +}
> > +#else
> > +static int handle_vector_misaligned_load(struct pt_regs *regs)
> > +{
> > + return -1;
> > +}
> > +#endif
> > +
> > +static int handle_scalar_misaligned_load(struct pt_regs *regs)
> > {
> > union reg_data val;
> > unsigned long epc = regs->epc;
> > @@ -435,7 +461,7 @@ int handle_misaligned_load(struct pt_regs *regs)
> > return 0;
> > }
> >
> > -int handle_misaligned_store(struct pt_regs *regs)
> > +static int handle_scalar_misaligned_store(struct pt_regs *regs)
> > {
> > union reg_data val;
> > unsigned long epc = regs->epc;
> > @@ -526,6 +552,86 @@ int handle_misaligned_store(struct pt_regs *regs)
> > return 0;
> > }
> >
> > +int handle_misaligned_load(struct pt_regs *regs)
> > +{
> > + unsigned long epc = regs->epc;
> > + unsigned long insn;
> > +
> > + if (IS_ENABLED(CONFIG_RISCV_VECTOR_MISALIGNED)) {
> > + if (get_insn(regs, epc, &insn))
> > + return -1;
> > +
> > + if (insn_is_vector(insn))
> > + return handle_vector_misaligned_load(regs);
> > + }
> > +
> > + if (IS_ENABLED(CONFIG_RISCV_SCALAR_MISALIGNED))
> > + return handle_scalar_misaligned_load(regs);
> > +
> > + return -1;
> > +}
> > +
> > +int handle_misaligned_store(struct pt_regs *regs)
> > +{
> > + if (IS_ENABLED(CONFIG_RISCV_SCALAR_MISALIGNED))
> > + return handle_scalar_misaligned_store(regs);
> > +
> > + return -1;
> > +}
> > +
> > +#ifdef CONFIG_RISCV_VECTOR_MISALIGNED
> > +void check_vector_unaligned_access_emulated(struct work_struct *work __always_unused)
> > +{
> > + long *mas_ptr = this_cpu_ptr(&vector_misaligned_access);
> > + unsigned long tmp_var;
> > +
> > + *mas_ptr = RISCV_HWPROBE_VECTOR_MISALIGNED_UNKNOWN;
> > +
> > + kernel_vector_begin();
> > + __asm__ __volatile__ (
> > + ".balign 4\n\t"
> > + ".option push\n\t"
> > + ".option arch, +zve32x\n\t"
> > + " vsetivli zero, 1, e16, m1, ta, ma\n\t" // Vectors of 16b
> > + " vle16.v v0, (%[ptr])\n\t" // Load bytes
> > + ".option pop\n\t"
> > + : : [ptr] "r" ((u8 *)&tmp_var + 1) : "v0");
> > + kernel_vector_end();
> > +
> > + if (*mas_ptr == RISCV_HWPROBE_VECTOR_MISALIGNED_UNKNOWN)
> > + *mas_ptr = RISCV_HWPROBE_VECTOR_MISALIGNED_SLOW;
>
> I'm still not a fan of this "if we don't know, say it's slow" thing.
> Most consumers of this key will do something like "if
> (misaligned_access == FAST) install_fast_functions()", and that'll
> work here too. But for usermode that really cares and is trying to
> interpret the other values, they're now forced to treat SLOW and
> UNKNOWN as the same value, since we sometimes return SLOW when we mean
> UNKNOWN. It seems better to me if the kernel is honestly reporting the
> information, and then usermode can extract real meaning from each of
> these values to the extent they care to.

I'm a massive idiot. I thought I had changed it to be UNKNOWN and not
SLOW before I sent it.
I'm very sorry.

I'll send a V7 ; ( if necessary.

Thanks,
Jesse Taube