Re: [PATCH v11 2/4] riscv: Include riscv_set_icache_flush_ctx prctl

Samuel Holland
2024-03-12T18:01:00Z

On 2024-02-12 5:36 PM, Charlie Jenkins wrote:
> Support new prctl with key PR_RISCV_SET_ICACHE_FLUSH_CTX to enable
> optimization of cross modifying code. This prctl enables userspace code
> to use icache flushing instructions such as fence.i with the guarantee
> that the icache will continue to be clean after thread migration.
>
> Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
> Reviewed-by: Atish Patra <atishp@rivosinc.com>
> Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> ---
> arch/riscv/include/asm/mmu.h | 2 +
> arch/riscv/include/asm/processor.h | 12 +++++
> arch/riscv/include/asm/switch_to.h | 23 ++++++++
> arch/riscv/mm/cacheflush.c | 105 +++++++++++++++++++++++++++++++++++++
> arch/riscv/mm/context.c | 18 +++++--
> include/uapi/linux/prctl.h | 6 +++
> kernel/sys.c | 6 +++
> 7 files changed, 167 insertions(+), 5 deletions(-)
>
> diff --git a/arch/riscv/include/asm/mmu.h b/arch/riscv/include/asm/mmu.h
> index 355504b37f8e..60be458e94da 100644
> --- a/arch/riscv/include/asm/mmu.h
> +++ b/arch/riscv/include/asm/mmu.h
> @@ -19,6 +19,8 @@ typedef struct {
> #ifdef CONFIG_SMP
> /* A local icache flush is needed before user execution can resume. */
> cpumask_t icache_stale_mask;
> + /* Force local icache flush on all migrations. */
> + bool force_icache_flush;
> #endif
> #ifdef CONFIG_BINFMT_ELF_FDPIC
> unsigned long exec_fdpic_loadmap;
> diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
> index a8509cc31ab2..46c5c3b91165 100644
> --- a/arch/riscv/include/asm/processor.h
> +++ b/arch/riscv/include/asm/processor.h
> @@ -69,6 +69,7 @@
> #endif
>
> #ifndef __ASSEMBLY__
> +#include <linux/cpumask.h>
>
> struct task_struct;
> struct pt_regs;
> @@ -123,6 +124,14 @@ struct thread_struct {
> struct __riscv_v_ext_state vstate;
> unsigned long align_ctl;
> struct __riscv_v_ext_state kernel_vstate;
> +#ifdef CONFIG_SMP
> + /* A local icache flush is needed before user execution can resume on one of these cpus. */
> + cpumask_t icache_stale_mask;
> + /* Regardless of the icache_stale_mask, flush the icache on migration */
> + bool force_icache_flush;
> + /* A forced icache flush is not needed if migrating to the previous cpu. */
> + unsigned int prev_cpu;
> +#endif
> };
>
> /* Whitelist the fstate from the task_struct for hardened usercopy */
> @@ -184,6 +193,9 @@ extern int set_unalign_ctl(struct task_struct *tsk, unsigned int val);
> #define GET_UNALIGN_CTL(tsk, addr) get_unalign_ctl((tsk), (addr))
> #define SET_UNALIGN_CTL(tsk, val) set_unalign_ctl((tsk), (val))
>
> +#define RISCV_SET_ICACHE_FLUSH_CTX(arg1, arg2) riscv_set_icache_flush_ctx(arg1, arg2)
> +extern int riscv_set_icache_flush_ctx(unsigned long ctx, unsigned long per_thread);
> +
> #endif /* __ASSEMBLY__ */
>
> #endif /* _ASM_RISCV_PROCESSOR_H */
> diff --git a/arch/riscv/include/asm/switch_to.h b/arch/riscv/include/asm/switch_to.h
> index 7efdb0584d47..7594df37cc9f 100644
> --- a/arch/riscv/include/asm/switch_to.h
> +++ b/arch/riscv/include/asm/switch_to.h
> @@ -8,6 +8,7 @@
>
> #include <linux/jump_label.h>
> #include <linux/sched/task_stack.h>
> +#include <linux/mm_types.h>
> #include <asm/vector.h>
> #include <asm/cpufeature.h>
> #include <asm/processor.h>
> @@ -72,14 +73,36 @@ static __always_inline bool has_fpu(void) { return false; }
> extern struct task_struct *__switch_to(struct task_struct *,
> struct task_struct *);
>
> +static inline bool switch_to_should_flush_icache(struct task_struct *task)
> +{
> +#ifdef CONFIG_SMP
> + bool stale_mm = task->mm && task->mm->context.force_icache_flush;
> + bool stale_thread = task->thread.force_icache_flush;
> + bool thread_migrated = smp_processor_id() != task->thread.prev_cpu;
> +
> + return thread_migrated && (stale_mm || stale_thread);
> +#else
> + return false;
> +#endif
> +}
> +
> +#ifdef CONFIG_SMP
> +#define __set_prev_cpu(thread) ((thread).prev_cpu = smp_processor_id())
> +#else
> +#define __set_prev_cpu(thread)
> +#endif
> +
> #define switch_to(prev, next, last) \
> do { \
> struct task_struct *__prev = (prev); \
> struct task_struct *__next = (next); \
> + __set_prev_cpu(__prev->thread); \
> if (has_fpu()) \
> __switch_to_fpu(__prev, __next); \
> if (has_vector()) \
> __switch_to_vector(__prev, __next); \
> + if (switch_to_should_flush_icache(__next)) \
> + local_flush_icache_all(); \
> ((last) = __switch_to(__prev, __next)); \
> } while (0)
>
> diff --git a/arch/riscv/mm/cacheflush.c b/arch/riscv/mm/cacheflush.c
> index 55a34f2020a8..6513a0ab8655 100644
> --- a/arch/riscv/mm/cacheflush.c
> +++ b/arch/riscv/mm/cacheflush.c
> @@ -5,6 +5,7 @@
>
> #include <linux/acpi.h>
> #include <linux/of.h>
> +#include <linux/prctl.h>
> #include <asm/acpi.h>
> #include <asm/cacheflush.h>
>
> @@ -152,3 +153,107 @@ void __init riscv_init_cbo_blocksizes(void)
> if (cboz_block_size)
> riscv_cboz_block_size = cboz_block_size;
> }
> +
> +/**
> + * riscv_set_icache_flush_ctx() - Enable/disable icache flushing instructions in
> + * userspace.
> + * @ctx: Set the type of icache flushing instructions permitted/prohibited in
> + * userspace. Supported values described below.
> + *
> + * Supported values for ctx:
> + *
> + * * %PR_RISCV_CTX_SW_FENCEI_ON: Allow fence.i in user space.
> + *
> + * * %PR_RISCV_CTX_SW_FENCEI_OFF: Disallow fence.i in user space. All threads in
> + * a process will be affected when ``scope == PR_RISCV_SCOPE_PER_PROCESS``.
> + * Therefore, caution must be taken; use this flag only when you can guarantee
> + * that no thread in the process will emit fence.i from this point onward.
> + *
> + * @scope: Set scope of where icache flushing instructions are allowed to be
> + * emitted. Supported values described below.
> + *
> + * Supported values for scope:
> + *
> + * * %PR_RISCV_SCOPE_PER_PROCESS: Ensure the icache of any thread in this process
> + * is coherent with instruction storage upon
> + * migration.
> + *
> + * * %PR_RISCV_SCOPE_PER_THREAD: Ensure the icache of the current thread is
> + * coherent with instruction storage upon
> + * migration.
> + *
> + * When ``scope == PR_RISCV_SCOPE_PER_PROCESS``, all threads in the process are
> + * permitted to emit icache flushing instructions. Whenever any thread in the
> + * process is migrated, the corresponding hart's icache will be guaranteed to be
> + * consistent with instruction storage. This does not enforce any guarantees
> + * outside of migration. If a thread modifies an instruction that another thread
> + * may attempt to execute, the other thread must still emit an icache flushing
> + * instruction before attempting to execute the potentially modified
> + * instruction. This must be performed by the user-space program.
> + *
> + * In per-thread context (eg. ``scope == PR_RISCV_SCOPE_PER_THREAD``) only the
> + * thread calling this function is permitted to emit icache flushing
> + * instructions. When the thread is migrated, the corresponding hart's icache
> + * will be guaranteed to be consistent with instruction storage.
> + *
> + * On kernels configured without SMP, this function is a nop as migrations
> + * across harts will not occur.
> + */
> +int riscv_set_icache_flush_ctx(unsigned long ctx, unsigned long scope)
> +{
> +#ifdef CONFIG_SMP
> + switch (ctx) {
> + case PR_RISCV_CTX_SW_FENCEI_ON:
> + switch (scope) {
> + case PR_RISCV_SCOPE_PER_PROCESS:
> + current->mm->context.force_icache_flush = true;
> + break;
> + case PR_RISCV_SCOPE_PER_THREAD:
> + current->thread.force_icache_flush = true;
> + break;
> + default:
> + return -EINVAL;
> + }
> + break;
> + case PR_RISCV_CTX_SW_FENCEI_OFF:
> + cpumask_t *mask;
> +
> + switch (scope) {
> + case PR_RISCV_SCOPE_PER_PROCESS:
> + bool stale_cpu;
> +
> + current->mm->context.force_icache_flush = false;
> +
> + /*
> + * Mark every other hart's icache as needing a flush for
> + * this MM. Maintain the previous value of the current
> + * cpu to handle the case when this function is called
> + * concurrently on different harts.
> + */
> + mask = &current->mm->context.icache_stale_mask;
> + stale_cpu = cpumask_test_cpu(smp_processor_id(), mask);
> +
> + cpumask_setall(mask);
> + assign_bit(cpumask_check(smp_processor_id()), cpumask_bits(mask), stale_cpu);
> + break;
> + case PR_RISCV_SCOPE_PER_THREAD:
> + current->thread.force_icache_flush = false;
> +
> + /*
> + * Mark every other hart's icache as needing a flush for
> + * this thread.
> + */
> + mask = &current->thread.icache_stale_mask;
> + cpumask_setall(mask);
> + cpumask_clear_cpu(smp_processor_id(), mask);

I don't think we need to maintain another cpumask for every thread (and check it
at every mm switch) just to optimize this prctl() call.
PR_RISCV_CTX_SW_FENCEI_OFF is unlikely to be in anything's hot path. It should
be sufficient for both scopes to just do:

cpumask_setall(&current->mm->context.icache_stale_mask);

and accept the extra one-time icache flushes.

Otherwise this looks good to me.

Regards,
Samuel

> + break;
> + default:
> + return -EINVAL;
> + }
> + break;
> + default:
> + return -EINVAL;
> + }
> +#endif
> + return 0;
> +}
> diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
> index 217fd4de6134..2eb13b89cced 100644
> --- a/arch/riscv/mm/context.c
> +++ b/arch/riscv/mm/context.c
> @@ -15,6 +15,7 @@
> #include <asm/tlbflush.h>
> #include <asm/cacheflush.h>
> #include <asm/mmu_context.h>
> +#include <asm/switch_to.h>
>
> #ifdef CONFIG_MMU
>
> @@ -297,21 +298,28 @@ static inline void set_mm(struct mm_struct *prev,
> *
> * The "cpu" argument must be the current local CPU number.
> */
> -static inline void flush_icache_deferred(struct mm_struct *mm, unsigned int cpu)
> +static inline void flush_icache_deferred(struct mm_struct *mm, unsigned int cpu,
> + struct task_struct *task)
> {
> #ifdef CONFIG_SMP
> cpumask_t *mask = &mm->context.icache_stale_mask;
>
> - if (cpumask_test_cpu(cpu, mask)) {
> + if (cpumask_test_and_clear_cpu(cpu, mask) ||
> + (task && cpumask_test_and_clear_cpu(cpu, &task->thread.icache_stale_mask))) {
> cpumask_clear_cpu(cpu, mask);
> +
> /*
> * Ensure the remote hart's writes are visible to this hart.
> * This pairs with a barrier in flush_icache_mm.
> */
> smp_mb();
> - local_flush_icache_all();
> - }
>
> + /*
> + * If cache will be flushed in switch_to, no need to flush here.
> + */
> + if (!(task && switch_to_should_flush_icache(task)))
> + local_flush_icache_all();
> + }
> #endif
> }
>
> @@ -332,5 +340,5 @@ void switch_mm(struct mm_struct *prev, struct mm_struct *next,
>
> set_mm(prev, next, cpu);
>
> - flush_icache_deferred(next, cpu);
> + flush_icache_deferred(next, cpu, task);
> }
> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> index 370ed14b1ae0..524d546d697b 100644
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -306,4 +306,10 @@ struct prctl_mm_map {
> # define PR_RISCV_V_VSTATE_CTRL_NEXT_MASK 0xc
> # define PR_RISCV_V_VSTATE_CTRL_MASK 0x1f
>
> +#define PR_RISCV_SET_ICACHE_FLUSH_CTX 71
> +# define PR_RISCV_CTX_SW_FENCEI_ON 0
> +# define PR_RISCV_CTX_SW_FENCEI_OFF 1
> +# define PR_RISCV_SCOPE_PER_PROCESS 0
> +# define PR_RISCV_SCOPE_PER_THREAD 1
> +
> #endif /* _LINUX_PRCTL_H */
> diff --git a/kernel/sys.c b/kernel/sys.c
> index e219fcfa112d..69afdd8b430f 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -146,6 +146,9 @@
> #ifndef RISCV_V_GET_CONTROL
> # define RISCV_V_GET_CONTROL() (-EINVAL)
> #endif
> +#ifndef RISCV_SET_ICACHE_FLUSH_CTX
> +# define RISCV_SET_ICACHE_FLUSH_CTX(a, b) (-EINVAL)
> +#endif
>
> /*
> * this is where the system-wide overflow UID and GID are defined, for
> @@ -2743,6 +2746,9 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
> case PR_RISCV_V_GET_CONTROL:
> error = RISCV_V_GET_CONTROL();
> break;
> + case PR_RISCV_SET_ICACHE_FLUSH_CTX:
> + error = RISCV_SET_ICACHE_FLUSH_CTX(arg2, arg3);
> + break;
> default:
> error = -EINVAL;
> break;
>