Re: [PATCH v10 1/2] riscv: Include riscv_set_icache_flush_ctx prctl

From: Samuel Holland
Date: Wed Feb 07 2024 - 13:55:51 EST

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Hi Charlie,

On 2024-01-24 10:23 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@xxxxxxxxxxxx>
> Reviewed-by: Atish Patra <atishp@xxxxxxxxxxxx>
> ---
> arch/riscv/include/asm/mmu.h | 2 ++
> arch/riscv/include/asm/processor.h | 7 ++++
> arch/riscv/include/asm/switch_to.h | 13 ++++++++
> arch/riscv/mm/cacheflush.c | 67 ++++++++++++++++++++++++++++++++++++++
> arch/riscv/mm/context.c | 13 ++++++--
> include/uapi/linux/prctl.h | 6 ++++
> kernel/sys.c | 6 ++++
> 7 files changed, 111 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 a8509cc31ab2..816cdc2395f4 100644
> --- a/arch/riscv/include/asm/processor.h
> +++ b/arch/riscv/include/asm/processor.h
> @@ -123,6 +123,10 @@ struct thread_struct {
> struct __riscv_v_ext_state vstate;
> unsigned long align_ctl;
> struct __riscv_v_ext_state kernel_vstate;
> +#ifdef CONFIG_SMP
> + bool force_icache_flush;
> + unsigned int prev_cpu;
> +#endif
> };
>
> /* Whitelist the fstate from the task_struct for hardened usercopy */
> @@ -184,6 +188,9 @@ 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..ac99ab64499c 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 +73,26 @@ 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)
> +{
> + bool stale_mm = task->mm && (task->mm->context.force_icache_flush);

No parentheses needed here.

> + bool stale_thread = task->thread.force_icache_flush;
> + bool thread_migrated = smp_processor_id() != task->thread.prev_cpu;

This fails to compile when SMP is disabled:

In file included from arch/riscv/kernel/ptrace.c:14:
/arch/riscv/include/asm/switch_to.h:78:49: error: no member named
'force_icache_flush' in 'mm_context_t'
bool stale_mm = task->mm && (task->mm->context.force_icache_flush);
~~~~~~~~~~~~~~~~~ ^
/arch/riscv/include/asm/switch_to.h:79:35: error: no member named
'force_icache_flush' in 'struct thread_struct'
bool stale_thread = task->thread.force_icache_flush;
~~~~~~~~~~~~ ^
/arch/riscv/include/asm/switch_to.h:80:60: error: no member named 'prev_cpu' in
'struct thread_struct'
bool thread_migrated = smp_processor_id() != task->thread.prev_cpu;
~~~~~~~~~~~~ ^
CC arch/riscv/kernel/sys_hwprobe.o

> +
> + return thread_migrated && (stale_mm || stale_thread);
> +}
> +
> #define switch_to(prev, next, last) \
> do { \
> struct task_struct *__prev = (prev); \
> struct task_struct *__next = (next); \
> + __next->thread.prev_cpu = smp_processor_id(); \

Shouldn't this instead update __prev (the task being scheduled out)?

> 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..2d5e1575f6c1 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 +153,69 @@ 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 userspace.
> + *
> + * * %PR_RISCV_CTX_SW_FENCEI_OFF: Disallow fence.i in userspace. All threads in
> + * a process will be affected when ``scope == PR_RISCV_SCOPE_PER_PROCESS``.
> + * Therefore, caution must be taken -- only use this flag 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. Note 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 userspace 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;
> + }

At some point you dropped the implementation for PR_RISCV_CTX_SW_FENCEI_OFF.
It's still documented, so I don't think that was intentional.

Also, PR_RISCV_CTX_SW_FENCEI_OFF needs to mark the icache on all other CPUs as
stale, as in flush_icache_mm(). Consider the following sequence:
1. Migrate from CPU A to CPU B
2. CMODX on CPU B
3. fence.i in userspace on CPU B
4. prctl(PR_RISCV_CTX_SW_FENCEI_OFF) on CPU B
5. Migrate back to CPU A -- force_icache_flush is false, no flush happens
6. Execute the modified code -- fails, because CPU A's I$ never got flushed

> + }
> +#endif
> + return 0;

This makes the prtcl() call silently succeed for invalid ctx values, whereas it
should fail for those instead.

Regards,
Samuel

> +}
> diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
> index 217fd4de6134..5cabbc449080 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
>
> @@ -297,7 +298,8 @@ 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;
> @@ -309,7 +311,12 @@ static inline void flush_icache_deferred(struct mm_struct *mm, unsigned int cpu)
> * 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 +339,5 @@ 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;
>

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