On Wed, Dec 13, 2023 at 2:48 PM Charlie Jenkins <charlie@xxxxxxxxxxxx> 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.
>
Do we need a CLEAR option as well ? I am wondering if there is a use case where
an application wants the prctl for only certain duration but may
invoke fence.i multiple times (syscall will still not be a good option
for them.)
> Signed-off-by: Charlie Jenkins <charlie@xxxxxxxxxxxx>
> ---
> arch/riscv/include/asm/mmu.h | 2 ++
> arch/riscv/include/asm/processor.h | 6 ++++++
> arch/riscv/mm/cacheflush.c | 37 +++++++++++++++++++++++++++++++++++++
> arch/riscv/mm/context.c | 8 +++++---
> include/uapi/linux/prctl.h | 3 +++
> kernel/sys.c | 6 ++++++
> 6 files changed, 59 insertions(+), 3 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 f19f861cda54..7eda6c75e0f2 100644
> --- a/arch/riscv/include/asm/processor.h
> +++ b/arch/riscv/include/asm/processor.h
> @@ -84,6 +84,9 @@ struct thread_struct {
> unsigned long vstate_ctrl;
> struct __riscv_v_ext_state vstate;
> unsigned long align_ctl;
> +#ifdef CONFIG_SMP
> + bool force_icache_flush;
> +#endif
> };
>
> /* Whitelist the fstate from the task_struct for hardened usercopy */
> @@ -145,6 +148,9 @@ 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/mm/cacheflush.c b/arch/riscv/mm/cacheflush.c
> index 55a34f2020a8..3b2bf8256a10 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 +153,39 @@ void __init riscv_init_cbo_blocksizes(void)
> if (cboz_block_size)
> riscv_cboz_block_size = cboz_block_size;
> }
> +
> +/**
> + * riscv_set_icache_flush_ctx() - Enable userspace to emit icache flushing instructions.
> + * @ctx: Sets the type of context
> + * - PR_RISCV_CTX_SW_FENCEI: Allow fence.i in userspace. Another fence.i will
> + * emitted on thread/process migration.
> + * @per_thread: When set to 0, will use the default behavior of setting the
> + * icache flush context per process. When set to 1, will use a per thread
> + * context.
> + *
> + * When in per-process context, there may be multiple threads using the same mm.
> + * Therefore, the icache can never be assumed clean when. Multiple threads in
> + * the process may modify instructions in the mm concurrently.
> + *
> + * In per-thread context, it can be assumed that all modifications to
> + * instructions in memory will be performed by this thread. When the thread is
> + * migrated the icache will be flushed.
> + *
> + */
> +int riscv_set_icache_flush_ctx(unsigned long ctx, unsigned long per_thread)
> +{
> +#ifdef CONFIG_SMP
> + switch (ctx) {
> + case PR_RISCV_CTX_SW_FENCEI:
> + if (per_thread)
> + current->thread.force_icache_flush = true;
> + else
> + current->mm->context.force_icache_flush = true;
> + break;
> +
> + default:
> + break;
> + }
> +#endif
> + return 0;
> +}
> diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
> index 217fd4de6134..0146c61be0ab 100644
> --- a/arch/riscv/mm/context.c
> +++ b/arch/riscv/mm/context.c
> @@ -297,12 +297,14 @@ 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_cpu(cpu, mask) || mm->context.force_icache_flush ||
> + (task && task->thread.force_icache_flush)) {
> cpumask_clear_cpu(cpu, mask);
> /*
> * Ensure the remote hart's writes are visible to this hart.
> @@ -332,5 +334,5 @@ 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..472801ea78cc 100644
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -306,4 +306,7 @@ 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 0
> +
> #endif /* _LINUX_PRCTL_H */
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 420d9cb9cc8e..e806a8a67c36 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
> @@ -2739,6 +2742,9 @@ 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;
>
> --
> 2.43.0
>
>
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--
Regards,
Atish
