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

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On Thu, Jul 25, 2024 at 12:58 PM Jesse Taube <jesse@xxxxxxxxxxxx> wrote:
>
> 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.

No worries at all. You are competent.

Seems like we can simply remove this conditional.

There was also a spot (I think in the next patch) where you compare
directly against == SLOW, which we can change to != UNKNOWN.
-Evan

>
> Thanks,
> Jesse Taube





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