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Date: Thu, 18 Jan 2024 15:16:57 -0800
From: Charlie Jenkins <charlie@rivosinc.com>
To: Alexandre Ghiti <alex@ghiti.fr>
Cc: Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>, Jonathan Corbet <corbet@lwn.net>,
	Conor Dooley <conor.dooley@microchip.com>,
	=?iso-8859-1?Q?Cl=E9ment_L=E9ger?= <cleger@rivosinc.com>,
	Atish Patra <atishp@atishpatra.org>,
	Randy Dunlap <rdunlap@infradead.org>,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	linux-doc@vger.kernel.org, Atish Patra <atishp@rivosinc.com>
Subject: Re: [PATCH v8 1/2] riscv: Include riscv_set_icache_flush_ctx prctl
Message-ID: <Zamxac0S7JnPmJdD@ghost>
References: <20240116-fencei-v8-0-43a42134f021@rivosinc.com>
 <20240116-fencei-v8-1-43a42134f021@rivosinc.com>
 <bb0e976d-7475-4c42-b87e-bf1af389ba5f@ghiti.fr>
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In-Reply-To: <bb0e976d-7475-4c42-b87e-bf1af389ba5f@ghiti.fr>

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@rivosinc.com>
> > Reviewed-by: Atish Patra <atishp@rivosinc.com>
> > ---
> >   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.

> 
> 
> > +}
> > +
> >   #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.

- 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;
> >