Re: [PATCH v2 7/8] riscv: report misaligned accesses emulation to hwprobe

[Evan Green]

On Wed, Oct 4, 2023 at 8:14 AM Clément Léger <cleger@xxxxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxx>
> ---
>  arch/riscv/include/asm/cpufeature.h  | 18 +++++++++
>  arch/riscv/kernel/cpufeature.c       |  4 ++
>  arch/riscv/kernel/smpboot.c          |  2 +-
>  arch/riscv/kernel/traps_misaligned.c | 56 ++++++++++++++++++++++++++++
>  4 files changed, 79 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/include/asm/cpufeature.h b/arch/riscv/include/asm/cpufeature.h
> index d0345bd659c9..e4ae6af51876 100644
> --- a/arch/riscv/include/asm/cpufeature.h
> +++ b/arch/riscv/include/asm/cpufeature.h
> @@ -32,4 +32,22 @@ extern struct riscv_isainfo hart_isa[NR_CPUS];
>
>  void check_unaligned_access(int cpu);
>
> +#ifdef CONFIG_RISCV_MISALIGNED
> +bool unaligned_ctl_available(void);
> +bool check_unaligned_access_emulated(int cpu);
> +void unaligned_emulation_finish(void);
> +#else
> +static inline bool unaligned_ctl_available(void)
> +{
> +       return false;
> +}
> +
> +static inline bool check_unaligned_access_emulated(int cpu)
> +{
> +       return false;
> +}
> +
> +static inline void unaligned_emulation_finish(void) {}
> +#endif
> +
>  #endif
> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> index 356e5677eeb1..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");
> @@ -648,6 +651,7 @@ void check_unaligned_access(int cpu)
>  static int __init check_unaligned_access_boot_cpu(void)
>  {
>         check_unaligned_access(0);
> +       unaligned_emulation_finish();
>         return 0;
>  }
>
> diff --git a/arch/riscv/kernel/smpboot.c b/arch/riscv/kernel/smpboot.c
> index 1b8da4e40a4d..5d9858d6ad26 100644
> --- a/arch/riscv/kernel/smpboot.c
> +++ b/arch/riscv/kernel/smpboot.c
> @@ -245,8 +245,8 @@ asmlinkage __visible void smp_callin(void)
>         riscv_ipi_enable();
>
>         numa_add_cpu(curr_cpuid);
> -       set_cpu_online(curr_cpuid, 1);
>         check_unaligned_access(curr_cpuid);
> +       set_cpu_online(curr_cpuid, 1);
>
>         if (has_vector()) {
>                 if (riscv_v_setup_vsize())
> diff --git a/arch/riscv/kernel/traps_misaligned.c b/arch/riscv/kernel/traps_misaligned.c
> index b5fb1ff078e3..d99b95084b6c 100644
> --- a/arch/riscv/kernel/traps_misaligned.c
> +++ b/arch/riscv/kernel/traps_misaligned.c
> @@ -14,6 +14,8 @@
>  #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
> @@ -396,6 +398,8 @@ union reg_data {
>         u64 data_u64;
>  };
>
> +static bool unaligned_ctl __read_mostly;
> +
>  /* sysctl hooks */
>  int unaligned_enabled __read_mostly = 1;       /* Enabled by default */
>
> @@ -409,6 +413,8 @@ int handle_misaligned_load(struct pt_regs *regs)
>
>         perf_sw_event(PERF_COUNT_SW_ALIGNMENT_FAULTS, 1, regs, addr);
>
> +       *this_cpu_ptr(&misaligned_access_speed) = RISCV_HWPROBE_MISALIGNED_EMULATED;
> +
>         if (!unaligned_enabled)
>                 return -1;
>
> @@ -585,3 +591,53 @@ int handle_misaligned_store(struct pt_regs *regs)
>
>         return 0;
>  }
> +
> +bool check_unaligned_access_emulated(int cpu)
> +{
> +       long *mas_ptr = per_cpu_ptr(&misaligned_access_speed, cpu);
> +       unsigned long tmp_var, tmp_val;
> +       bool misaligned_emu_detected;
> +
> +       *mas_ptr = RISCV_HWPROBE_MISALIGNED_UNKNOWN;
> +
> +       __asm__ __volatile__ (
> +               "       "REG_L" %[tmp], 1(%[ptr])\n"
> +               : [tmp] "=r" (tmp_val) : [ptr] "r" (&tmp_var) : "memory");
> +
> +       misaligned_emu_detected = (*mas_ptr == RISCV_HWPROBE_MISALIGNED_EMULATED);
> +       /*
> +        * If unaligned_ctl is already set, this means that we detected that all
> +        * CPUS uses emulated misaligned access at boot time. If that changed
> +        * when hotplugging the new cpu, this is something we don't handle.
> +        */
> +       if (unlikely(unaligned_ctl && !misaligned_emu_detected)) {
> +               pr_crit("CPU misaligned accesses non homogeneous (expected all emulated)\n");
> +               while (true)
> +                       cpu_relax();

So the idea is to spin long enough that the
wait_for_completion(&cpu_running, 1000ms) times out? Maybe there
should be a wfi() in here as well so we're not just burning white hot.
Have you verified that if we get here, the CPU will also get taken
back down after the timeout? I wonder if __cpu_up() also needs a call
to stop the CPU, in the case where that wait_for_completion_timeout()
times out.

It also might be more intuitive to reorganize this such that the death
loop happens in smp_callin(), as check_unaligned_access_emulated() is
not a function you'd expect might sometimes never return.

-Evan