Re: [PATCH v8 1/2] riscv: Include riscv_set_icache_flush_ctx prctl
From: Alexandre Ghiti
Date: Fri Jan 19 2024 - 03:05:48 EST
On 19/01/2024 00:16, Charlie Jenkins wrote:
On Wed, Jan 17, 2024 at 09:42:43AM +0100, Alexandre Ghiti wrote:
Hi Charlie,
On 17/01/2024 03:54, 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 | 4 +++
arch/riscv/include/asm/mmu_context.h | 3 ++
arch/riscv/include/asm/processor.h | 7 ++++
arch/riscv/include/asm/switch_to.h | 17 +++++++++
arch/riscv/mm/cacheflush.c | 67 ++++++++++++++++++++++++++++++++++++
arch/riscv/mm/context.c | 14 ++++++--
include/uapi/linux/prctl.h | 6 ++++
kernel/sys.c | 6 ++++
8 files changed, 121 insertions(+), 3 deletions(-)
diff --git a/arch/riscv/include/asm/mmu.h b/arch/riscv/include/asm/mmu.h
index 355504b37f8e..f437b9a7f5b3 100644
--- a/arch/riscv/include/asm/mmu.h
+++ b/arch/riscv/include/asm/mmu.h
@@ -19,6 +19,10 @@ 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;
+ /* The most recent cpu a thread in this mm has been migrated to. */
+ unsigned int prev_cpu;
Why would we need a prev_cpu on mm? Why don't we only need to have a
prev_cpu per thread, i.e. per task_struct? It makes little sense to me to
have that on mm since it's shared by multiple threads (by definition), so
the prev_cpu on mm could be anything.
Yes you are correct, I will remove that.
#endif
#ifdef CONFIG_BINFMT_ELF_FDPIC
unsigned long exec_fdpic_loadmap;
diff --git a/arch/riscv/include/asm/mmu_context.h b/arch/riscv/include/asm/mmu_context.h
index 7030837adc1a..195a2e90f3f9 100644
--- a/arch/riscv/include/asm/mmu_context.h
+++ b/arch/riscv/include/asm/mmu_context.h
@@ -29,6 +29,9 @@ static inline int init_new_context(struct task_struct *tsk,
{
#ifdef CONFIG_MMU
atomic_long_set(&mm->context.id, 0);
+#endif
+#ifdef CONFIG_SMP
+ mm->context.prev_cpu = tsk->thread.prev_cpu;
#endif
return 0;
}
diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
index f19f861cda54..1cad05f579ad 100644
--- a/arch/riscv/include/asm/processor.h
+++ b/arch/riscv/include/asm/processor.h
@@ -84,6 +84,10 @@ struct thread_struct {
unsigned long vstate_ctrl;
struct __riscv_v_ext_state vstate;
unsigned long align_ctl;
+#ifdef CONFIG_SMP
+ bool force_icache_flush;
+ unsigned int prev_cpu;
+#endif
};
/* Whitelist the fstate from the task_struct for hardened usercopy */
@@ -145,6 +149,9 @@ 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 f90d8e42f3c7..6a94431dc193 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>
@@ -73,6 +74,17 @@ 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)
+{
+ unsigned int cpu = smp_processor_id();
+ bool stale_mm = task->mm && (task->mm->context.force_icache_flush &&
+ (cpu != task->mm->context.prev_cpu));
+ bool stale_thread = task->thread.force_icache_flush &&
+ (cpu != task->thread.prev_cpu);
+
+ return stale_mm || stale_thread;
Here the test would become:
return ((task->mm && task->mm->context.force_icache_flush) ||
task->thread.force_icache_flush) && (cpu != task->thread.prev_cpu);
And do we really need to check task->mm is not null?
mm can be null if the task is associated with a kernel thread. So the
check is necessary.
Yes, you're right, thanks
+}
+
#define switch_to(prev, next, last) \
do { \
struct task_struct *__prev = (prev); \
@@ -81,7 +93,12 @@ do { \
__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)); \
+ __next->thread.prev_cpu = smp_processor_id(); \
+ if (__next->mm) \
+ __next->mm->context.prev_cpu = smp_processor_id(); \
} while (0)
And just to make sure I understand this: you moved the test in switch_to()
because 2 threads with the same mm could be scheduled one after the other on
the same cpu and then switch_mm() would not be called?
Yes exactly.
Very good catch then!
Thanks,
Alex
- Charlie
#endif /* _ASM_RISCV_SWITCH_TO_H */
diff --git a/arch/riscv/mm/cacheflush.c b/arch/riscv/mm/cacheflush.c
index 55a34f2020a8..ff545f19f07a 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. When ``scope ==
+ * PR_RISCV_SCOPE_PER_PROCESS``, this will effect all threads in a 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;
+ }
+ }
+#endif
+ return 0;
+}
diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
index 217fd4de6134..b059dc0fae91 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
@@ -297,19 +298,26 @@ 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)) {
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 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;
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