Re: [PATCH v2 4/6] RISC-V: Detect unaligned vector accesses supported.

Andy Chiu
2024-06-24T05:34:22Z

On Fri, Jun 21, 2024 at 2:52 AM Evan Green <evan@rivosinc.com> wrote:
>
> On Thu, Jun 13, 2024 at 12:17 PM Jesse Taube <jesse@rivosinc.com> 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@rivosinc.com>
> > ---
> > 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
> > ---
> > arch/riscv/Kconfig | 41 +++++++
> > arch/riscv/include/asm/cpufeature.h | 7 +-
> > arch/riscv/include/asm/entry-common.h | 11 --
> > arch/riscv/include/asm/hwprobe.h | 2 +-
> > arch/riscv/include/asm/vector.h | 1 +
> > arch/riscv/include/uapi/asm/hwprobe.h | 5 +
> > arch/riscv/kernel/Makefile | 4 +-
> > arch/riscv/kernel/sys_hwprobe.c | 41 +++++++
> > arch/riscv/kernel/traps_misaligned.c | 119 ++++++++++++++++++++-
> > arch/riscv/kernel/unaligned_access_speed.c | 9 +-
> > arch/riscv/kernel/vector.c | 2 +-
> > 11 files changed, 221 insertions(+), 21 deletions(-)
> >
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index b94176e25be1..f12df0ca6c18 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -723,6 +723,12 @@ config RISCV_MISALIGNED
> > help
> > Embed support for emulating misaligned loads and stores.
> >
> > +config RISCV_VECTOR_MISALIGNED
> > + bool
> > + 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 +780,41 @@ 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_DETECT_VECTOR_UNALIGNED_ACCESS
> > + bool "Detect support for vector unaligned accesses"
> > + select RISCV_VECTOR_MISALIGNED
> > + help
> > + During boot, the kernel will detect if the system supports vector
> > + unaligned accesses.
> > +
> > +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.
> > +
> > +config CONFIG_RISCV_UNALIGNED_ACCESS_UNSUPPORTED
> > + bool "Assume the system does not support vector unaligned memory accesses"
> > + help
> > + Assume that the system does not support vector unaligned memory accesses.
> > + The kernel and userspace programs may run them successfully on systems
> > + that do support vector unaligned memory accesses.
> > +
> > +endchoice
> > +
> > endmenu # "Platform type"
> >
> > menu "Kernel features"
> > diff --git a/arch/riscv/include/asm/cpufeature.h b/arch/riscv/include/asm/cpufeature.h
> > index 347805446151..d0ea5921ab20 100644
> > --- a/arch/riscv/include/asm/cpufeature.h
> > +++ b/arch/riscv/include/asm/cpufeature.h
> > @@ -33,8 +33,8 @@ extern struct riscv_isainfo hart_isa[NR_CPUS];
> >
> > void riscv_user_isa_enable(void);
> >
> > -#if defined(CONFIG_RISCV_MISALIGNED)
> > bool check_unaligned_access_emulated_all_cpus(void);
> > +#if defined(CONFIG_RISCV_MISALIGNED)
> > void unaligned_emulation_finish(void);
> > bool unaligned_ctl_available(void);
> > DECLARE_PER_CPU(long, misaligned_access_speed);
> > @@ -45,6 +45,11 @@ static inline bool unaligned_ctl_available(void)
> > }
> > #endif
> >
> > +bool check_vector_unaligned_access_emulated_all_cpus(void);
> > +#if defined(CONFIG_RISCV_VECTOR_MISALIGNED)
> > +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 2293e535f865..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_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..99b0f91db9ee 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);
> > diff --git a/arch/riscv/include/uapi/asm/hwprobe.h b/arch/riscv/include/uapi/asm/hwprobe.h
> > index 023b7771d1b7..2fee870e41bb 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_VEC_MISALIGNED_PERF 8
> > +#define RISCV_HWPROBE_VEC_MISALIGNED_UNKNOWN 0
> > +#define RISCV_HWPROBE_VEC_MISALIGNED_SLOW 2
> > +#define RISCV_HWPROBE_VEC_MISALIGNED_FAST 3
> > +#define RISCV_HWPROBE_VEC_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 5b243d46f4b1..62ac19c029f1 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_MISALIGNED) += traps_misaligned.o
> > -obj-$(CONFIG_RISCV_MISALIGNED) += unaligned_access_speed.o
> > +obj-y += traps_misaligned.o
> > +obj-y += unaligned_access_speed.o
> > obj-$(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..c40df314058b 100644
> > --- a/arch/riscv/kernel/sys_hwprobe.c
> > +++ b/arch/riscv/kernel/sys_hwprobe.c
> > @@ -194,6 +194,43 @@ 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;
> > +
> > + if (IS_ENABLED(CONFIG_RISCV_UNALIGNED_ACCESS_UNSUPPORTED))
> > + return RISCV_HWPROBE_VEC_MISALIGNED_UNSUPPORTED;
> > +
> > + /* 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_VEC_MISALIGNED_UNKNOWN;
> > + break;
> > + }
> > + }
> > +
> > + if (perf == -1ULL)
> > + return RISCV_HWPROBE_VEC_MISALIGNED_UNKNOWN;
> > +
> > + return perf;
> > +}
> > +#else
> > +static u64 hwprobe_vec_misaligned(const struct cpumask *cpus)
> > +{
> > + if (IS_ENABLED(CONFIG_RISCV_UNALIGNED_ACCESS_UNSUPPORTED))
> > + return RISCV_HWPROBE_VEC_MISALIGNED_UNSUPPORTED;
> > +
> > + return RISCV_HWPROBE_VEC_MISALIGNED_UNKNOWN;
> > +}
> > +#endif
> > +
> > static void hwprobe_one_pair(struct riscv_hwprobe *pair,
> > const struct cpumask *cpus)
> > {
> > @@ -222,6 +259,10 @@ static void hwprobe_one_pair(struct riscv_hwprobe *pair,
> > pair->value = hwprobe_misaligned(cpus);
> > break;
> >
> > + case RISCV_HWPROBE_KEY_VEC_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 8fadbe00dd62..6f0264a8c9de 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_VEC_MISALIGNED_UNKNOWN) {
> > + *this_cpu_ptr(&vector_misaligned_access) = RISCV_HWPROBE_VEC_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 +552,85 @@ 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_MISALIGNED))
> > + return handle_scalar_misaligned_load(regs);
> > +
> > + return -1;
> > +}
> > +
> > +int handle_misaligned_store(struct pt_regs *regs)
> > +{
> > + if (IS_ENABLED(CONFIG_RISCV_MISALIGNED))
> > + return handle_scalar_misaligned_store(regs);
> > +
> > + return -1;
> > +}
> > +
> > +#ifdef CONFIG_RISCV_VECTOR_MISALIGNED
> > +static void check_vector_unaligned_access_emulated(struct work_struct *unused)
> > +{
> > + long *mas_ptr = this_cpu_ptr(&vector_misaligned_access);
> > + unsigned long tmp_var;
> > +
> > + *mas_ptr = RISCV_HWPROBE_VEC_MISALIGNED_UNKNOWN;
> > +
> > + local_irq_enable();
>
> Generally if a function is called with interrupts disabled there's a
> reason for it, like the system will crash if an interrupt fires during
> execution of this region. I haven't researched this, but to feel
> comfortable I'd want to know why interrupts were disabled on entry
> here, why it's safe to enable them now, and why it's safe to return
> from the function with them still enabled.
>
> I'm guessing this was added because may_use_simd() was blowing up for
> you without it. If that's the case, I think we'll need to reconcile
> that in a different way. From a quick glance at kernel_mode_vector.c,
> I was originally thinking may_use_simd() enforces this because there's

Current nesting support of kernel mode vector only allows softirq
context to use v context nesting on top of kernel thread's v context.
I did not expect the use of v in the interrupt handler but I am open
to discussions if it is needed.

The reason why may_use_simd() checks for irq_disabled() is that
local_bh_enable later in kernel_vector_end() expects running with irq
on. Although the success path of preemptible v does not require
!irq_disabled(), the kernel falls back to non-preemptible v
(get_cpu_vector_context) in some cases. First is the nesting softirq
case, or a rare case where an exception handler uses v in a kernel
fault that also uses v. The second is when memory allocation for
kernel_vstate fails. The fall back uses the non-preemptible v, so it
is linked to bh_*.

We can get rid of the last case by failing the launch of a kernel
thread when the allocation fails.

> only a single instance of current->thread.kernel_vstate. So if you
> tried to start a v context from an interrupt, you may land on top of
> another kernel user, and there's nowhere to save that kernel user's
> old state. But there does seem to be some support for nested V context
> associated with CONFIG_RISCV_ISA_V_PREEMPTIVE, so I'm a little
> confused. It seems like your options are to try and get running on
> each CPU in a different manner (such that you're not stuck in irq
> context by the time you first run), or try and dive into the kernel V
> context code to enable support for starting a V context in irq land.
>
>
> > + 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();
> > +}
> > +
> > +bool check_vector_unaligned_access_emulated_all_cpus(void)
> > +{
> > + int cpu;
> > + bool ret = true;
> > +
> > + if (!has_vector()) {
> > + for_each_online_cpu(cpu)
> > + per_cpu(vector_misaligned_access, cpu) = RISCV_HWPROBE_VEC_MISALIGNED_UNSUPPORTED;
> > + return false;
> > + }
> > +
> > + schedule_on_each_cpu(check_vector_unaligned_access_emulated);
> > +
> > + for_each_online_cpu(cpu)
> > + if (per_cpu(vector_misaligned_access, cpu)
> > + != RISCV_HWPROBE_VEC_MISALIGNED_SLOW)
> > + return false;
> > +
> > + return ret;
> > +}
> > +#else
> > +bool check_vector_unaligned_access_emulated_all_cpus(void)
> > +{
> > + return false;
> > +}
> > +#endif
> > +
> > +#ifdef CONFIG_RISCV_MISALIGNED
> > +
> > +static bool unaligned_ctl __read_mostly;
> > +
> > static void check_unaligned_access_emulated(struct work_struct *unused)
> > {
> > int cpu = smp_processor_id();
> > @@ -563,3 +668,9 @@ bool unaligned_ctl_available(void)
> > {
> > return unaligned_ctl;
> > }
> > +#else
> > +bool check_unaligned_access_emulated_all_cpus(void)
> > +{
> > + return false;
> > +}
> > +#endif
> > diff --git a/arch/riscv/kernel/unaligned_access_speed.c b/arch/riscv/kernel/unaligned_access_speed.c
> > index 70c1588fc353..c6106bd4a25a 100644
> > --- a/arch/riscv/kernel/unaligned_access_speed.c
> > +++ b/arch/riscv/kernel/unaligned_access_speed.c
> > @@ -19,7 +19,8 @@
> > #define MISALIGNED_BUFFER_ORDER get_order(MISALIGNED_BUFFER_SIZE)
> > #define MISALIGNED_COPY_SIZE ((MISALIGNED_BUFFER_SIZE / 2) - 0x80)
> >
> > -DEFINE_PER_CPU(long, misaligned_access_speed);
> > +DEFINE_PER_CPU(long, misaligned_access_speed) = RISCV_HWPROBE_MISALIGNED_UNKNOWN;
> > +DEFINE_PER_CPU(long, vector_misaligned_access) = RISCV_HWPROBE_VEC_MISALIGNED_UNSUPPORTED;
> >
> > #ifdef CONFIG_RISCV_PROBE_UNALIGNED_ACCESS
> > static cpumask_t fast_misaligned_access;
> > @@ -268,12 +269,18 @@ static int check_unaligned_access_all_cpus(void)
> >
> > if (riscv_has_extension_unlikely(RISCV_ISA_EXT_ZICCLSM)) {
> > for_each_online_cpu(cpu) {
> > +#ifdef CONFIG_RISCV_VECTOR_MISALIGNED
> > + per_cpu(vector_misaligned_access, cpu) = RISCV_HWPROBE_VEC_MISALIGNED_FAST;
> > +#endif
> > +#ifdef CONFIG_RISCV_MISALIGNED
> > per_cpu(misaligned_access_speed, cpu) = RISCV_HWPROBE_MISALIGNED_FAST;
> > +#endif
> > }
> > return 0;
> > }
> >
> > all_cpus_emulated = check_unaligned_access_emulated_all_cpus();
> > + check_vector_unaligned_access_emulated_all_cpus();
> >
> > #ifdef CONFIG_RISCV_PROBE_UNALIGNED_ACCESS
> > if (!all_cpus_emulated)
> > diff --git a/arch/riscv/kernel/vector.c b/arch/riscv/kernel/vector.c
> > index 682b3feee451..821818886fab 100644
> > --- a/arch/riscv/kernel/vector.c
> > +++ b/arch/riscv/kernel/vector.c
> > @@ -66,7 +66,7 @@ void __init riscv_v_setup_ctx_cache(void)
> > #endif
> > }
> >
> > -static bool insn_is_vector(u32 insn_buf)
> > +bool insn_is_vector(u32 insn_buf)
> > {
> > u32 opcode = insn_buf & __INSN_OPCODE_MASK;
> > u32 width, csr;
> > --
> > 2.43.0
> >

Thanks,
Andy