Re: [PATCH 6/7] riscv: report misaligned accesses emulation to hwprobe

[Atish Kumar Patra]

On Thu, Sep 28, 2023 at 9:52 AM Evan Green <evan@xxxxxxxxxxxx> wrote:
>
> On Thu, Sep 28, 2023 at 12:46 AM Clément Léger <cleger@xxxxxxxxxxxx> wrote:
> >
> >
> >
> > On 26/09/2023 23:57, Evan Green wrote:
> > > On Tue, Sep 26, 2023 at 8:03 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  |  6 +++
> > >>  arch/riscv/kernel/cpufeature.c       |  6 ++-
> > >>  arch/riscv/kernel/setup.c            |  1 +
> > >>  arch/riscv/kernel/traps_misaligned.c | 63 +++++++++++++++++++++++++++-
> > >>  4 files changed, 74 insertions(+), 2 deletions(-)
> > >>
> > >> diff --git a/arch/riscv/include/asm/cpufeature.h b/arch/riscv/include/asm/cpufeature.h
> > >> index d0345bd659c9..c1f0ef02cd7d 100644
> > >> --- a/arch/riscv/include/asm/cpufeature.h
> > >> +++ b/arch/riscv/include/asm/cpufeature.h
> > >> @@ -8,6 +8,7 @@
> > >>
> > >>  #include <linux/bitmap.h>
> > >>  #include <asm/hwcap.h>
> > >> +#include <asm/hwprobe.h>
> > >>
> > >>  /*
> > >>   * These are probed via a device_initcall(), via either the SBI or directly
> > >> @@ -32,4 +33,9 @@ extern struct riscv_isainfo hart_isa[NR_CPUS];
> > >>
> > >>  void check_unaligned_access(int cpu);
> > >>
> > >> +bool unaligned_ctl_available(void);
> > >> +
> > >> +bool check_unaligned_access_emulated(int cpu);
> > >> +void unaligned_emulation_finish(void);
> > >> +
> > >>  #endif
> > >> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> > >> index 1cfbba65d11a..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))
> > >
> > > This spot (referenced below).
> > >
> > >> +               return;
> > >> +
> > >>         page = alloc_pages(GFP_NOWAIT, get_order(MISALIGNED_BUFFER_SIZE));
> > >>         if (!page) {
> > >>                 pr_warn("Can't alloc pages to measure memcpy performance");
> > >> @@ -645,9 +648,10 @@ void check_unaligned_access(int cpu)
> > >>         __free_pages(page, get_order(MISALIGNED_BUFFER_SIZE));
> > >>  }
> > >>
> > >> -static int check_unaligned_access_boot_cpu(void)
> > >> +static int __init check_unaligned_access_boot_cpu(void)
> > >>  {
> > >>         check_unaligned_access(0);
> > >> +       unaligned_emulation_finish();
> > >>         return 0;
> > >>  }
> > >>
> > >> diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
> > >> index e600aab116a4..3af6ad4df7cf 100644
> > >> --- a/arch/riscv/kernel/setup.c
> > >> +++ b/arch/riscv/kernel/setup.c
> > >> @@ -26,6 +26,7 @@
> > >>  #include <asm/acpi.h>
> > >>  #include <asm/alternative.h>
> > >>  #include <asm/cacheflush.h>
> > >> +#include <asm/cpufeature.h>
> > >>  #include <asm/cpu_ops.h>
> > >>  #include <asm/early_ioremap.h>
> > >>  #include <asm/pgtable.h>
> > >> diff --git a/arch/riscv/kernel/traps_misaligned.c b/arch/riscv/kernel/traps_misaligned.c
> > >> index b5fb1ff078e3..fa81f6952fa4 100644
> > >> --- a/arch/riscv/kernel/traps_misaligned.c
> > >> +++ b/arch/riscv/kernel/traps_misaligned.c
> > >> @@ -9,11 +9,14 @@
> > >>  #include <linux/perf_event.h>
> > >>  #include <linux/irq.h>
> > >>  #include <linux/stringify.h>
> > >> +#include <linux/prctl.h>
> > >>
> > >>  #include <asm/processor.h>
> > >>  #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
> > >> @@ -396,8 +399,10 @@ union reg_data {
> > >>         u64 data_u64;
> > >>  };
> > >>
> > >> +static bool unaligned_ctl __read_mostly;
> > >> +
> > >>  /* sysctl hooks */
> > >> -int unaligned_enabled __read_mostly = 1;       /* Enabled by default */
> > >> +int unaligned_enabled __read_mostly;
> > >>
> > >>  int handle_misaligned_load(struct pt_regs *regs)
> > >>  {
> > >> @@ -412,6 +417,9 @@ int handle_misaligned_load(struct pt_regs *regs)
> > >>         if (!unaligned_enabled)
> > >>                 return -1;
> > >>
> > >> +       if (user_mode(regs) && (current->thread.align_ctl & PR_UNALIGN_SIGBUS))
> > >> +               return -1;
> > >> +
> > >>         if (get_insn(regs, epc, &insn))
> > >>                 return -1;
> > >>
> > >> @@ -511,6 +519,9 @@ int handle_misaligned_store(struct pt_regs *regs)
> > >>         if (!unaligned_enabled)
> > >>                 return -1;
> > >>
> > >> +       if (user_mode(regs) && (current->thread.align_ctl & PR_UNALIGN_SIGBUS))
> > >> +               return -1;
> > >> +
> > >>         if (get_insn(regs, epc, &insn))
> > >>                 return -1;
> > >>
> > >> @@ -585,3 +596,53 @@ int handle_misaligned_store(struct pt_regs *regs)
> > >>
> > >>         return 0;
> > >>  }
> > >> +
> > >> +bool check_unaligned_access_emulated(int cpu)
> > >> +{
> > >> +       unsigned long emulated = 1, tmp_var;
> > >> +
> > >> +       /* Use a fixup to detect if misaligned access triggered an exception */
> > >> +       __asm__ __volatile__ (
> > >> +               "1:\n"
> > >> +               "       "REG_L" %[tmp], 1(%[ptr])\n"
> > >> +               "       li %[emulated], 0\n"
> > >> +               "2:\n"
> > >> +               _ASM_EXTABLE(1b, 2b)
> > >> +       : [emulated] "+r" (emulated), [tmp] "=r" (tmp_var)
> > >> +       : [ptr] "r" (&tmp_var)
> > >> +       : "memory");
> > >> +
> > >> +       if (!emulated)
> > >> +               return false;
> > >> +
> > >> +       per_cpu(misaligned_access_speed, cpu) =
> > >> +               RISCV_HWPROBE_MISALIGNED_EMULATED;
> > >
> > > For tidiness, can we move the assignment of this per-cpu variable into
> > > check_unaligned_access(), at the spot I referenced above. That way
> > > people looking to see how this variable is set don't have to hunt
> > > through multiple locations.
> >
> > Agreed, that seems better.
> >
> > >
> > >> +
> > >> +       return true;
> > >> +}
> > >> +
> > >> +void __init unaligned_emulation_finish(void)
> > >> +{
> > >> +       int cpu;
> > >> +
> > >> +       /*
> > >> +        * We can only support PR_UNALIGN controls if all CPUs have misaligned
> > >> +        * accesses emulated since tasks requesting such control can run on any
> > >> +        * CPU.
> > >> +        */
> > >> +       for_each_possible_cpu(cpu) {
> > >> +               if (per_cpu(misaligned_access_speed, cpu) !=
> > >> +                   RISCV_HWPROBE_MISALIGNED_EMULATED) {
> > >> +                       goto out;
> > >> +               }
> > >> +       }
> > >> +       unaligned_ctl = true;
> > >

Note: You probably want to loop through the present cpu mask instead
of possible cpus.
Possible cpus list will have all the cpus listed in DT/ACPI. However,
all of them may not come up during the boot.
Hardware errata, different kernel configuration, incorrect DT are few
examples where possible may not match present cpumask.

> > > This doesn't handle the case where a CPU is hotplugged later that
> > > doesn't match with the others. You may want to add a patch that fails
> > > the onlining of that new CPU if unaligned_ctl is true and
> > > new_cpu.misaligned_access_speed != EMULATED.
> >
> > So actually, this will require a bit more plumbing as I realize the
> > switch to disable misalignment support is global. This switch should
> > only be disabled at boot which means I won't be able to disable it at
> > runtime (while hiotplugging a CPU) for CPU detection. There are multiple
> > ways to handle that:
> >
> > 1- Have a per-cpu switch for misalignment handling which would be
> > disabled only when detection is needed.
> >
> > 2- Assume that once detected at boot-time, emulation will not change.
> >
> > Not sure which one is better though. Advice are welcomed.
>
> If I gaze into my own crystal ball, my hope is that the Venn diagram
> of "systems that support hotplug" and "systems that still use software
> assist for misaligned access" is just two circles not touching. If
> people agree with that, then the safe thing to do is enforce it, by

In a sane world, this is probably true. But given that errats exists,
who knows what systems
we may end up with.

> failing to online new hotplugged CPUs that don't conform to
> misaligned_access_speed == EMULATED if unaligned_ctl is true. We would
> sacrifice some future flexibility by making this choice now though, so
> it requires buy-in for this particular crystal ball vision.
>
> -Evan