Subject: Re: [PATCH] add icswx support

From: Benjamin Herrenschmidt <[email-masked]>
Date: Sat, 24 Apr 2010 10:55:51 +1000

On Fri, 2010-04-23 at 17:04 -0500, Tseng-Hui (Frank) Lin wrote:
> Add Power7 icswx co-processor instruction support.

Please provide a -much- more detailed explanation of what it is, what it
does and why it requires hooking into the MMU context switch code. _I_
know these things but nobody else on the list does which limits the
ability of people to review your patch.

> Signed-off-by: Sonny Rao <[email-masked]>
> Signed-off-by: Tseng-Hui (Frank) Lin <[email-masked]>
> ---
>  arch/powerpc/include/asm/mmu-hash64.h  |    3 +
>  arch/powerpc/include/asm/mmu_context.h |    4 ++
>  arch/powerpc/include/asm/reg.h         |    3 +
>  arch/powerpc/mm/mmu_context_hash64.c   |   79
> ++++++++++++++++++++++++++++++++
>  4 files changed, 89 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/mmu-hash64.h
> b/arch/powerpc/include/asm/mmu-hash64.h
> index 2102b21..ba5727d 100644
> --- a/arch/powerpc/include/asm/mmu-hash64.h
> +++ b/arch/powerpc/include/asm/mmu-hash64.h
> @@ -421,6 +421,9 @@ typedef struct {
>  #ifdef CONFIG_PPC_SUBPAGE_PROT
>  	struct subpage_prot_table spt;
>  #endif /* CONFIG_PPC_SUBPAGE_PROT */
> +	unsigned long acop;
> +#define HASH64_MAX_PID (0xFFFF)
> +	unsigned int pid;

Please add a comment as to what those two fields are about, something
like acop; /* mask of enabled coprocessor types */ and pid /* pid
value used with coprocessors */ or something like that.

>  } mm_context_t;
>  
> 
> diff --git a/arch/powerpc/include/asm/mmu_context.h
> b/arch/powerpc/include/asm/mmu_context.h
> index 26383e0..d6c8841 100644
> --- a/arch/powerpc/include/asm/mmu_context.h
> +++ b/arch/powerpc/include/asm/mmu_context.h
> @@ -78,6 +78,10 @@ static inline void switch_mm(struct mm_struct *prev,
> struct mm_struct *next,
>  
>  #define deactivate_mm(tsk,mm)	do { } while (0)
>  
> +extern void switch_cop(struct mm_struct *next);
> +extern int  use_cop(unsigned long acop, struct task_struct *task);
              ^ remove that space
> +extern void disuse_cop(unsigned long acop, struct mm_struct *mm);

I'd prefer "drop" or "forget" :-)

> +
>  /*
>   * After we have set current->mm to a new value, this activates
>   * the context for the new mm so we see the new mappings.
> diff --git a/arch/powerpc/include/asm/reg.h
> b/arch/powerpc/include/asm/reg.h
> index 5572e86..30503f8 100644
> --- a/arch/powerpc/include/asm/reg.h
> +++ b/arch/powerpc/include/asm/reg.h
> @@ -516,6 +516,9 @@
>  #define SPRN_SIAR	780
>  #define SPRN_SDAR	781
>  
> +#define SPRN_ACOP	 31
> +#define SPRN_PID	 48
> +

Remove the definition of SPRN_PID from reg_booke.h to avoid a double
definition then.

>  #define SPRN_PA6T_MMCR0 795
>  #define   PA6T_MMCR0_EN0	0x0000000000000001UL
>  #define   PA6T_MMCR0_EN1	0x0000000000000002UL
> diff --git a/arch/powerpc/mm/mmu_context_hash64.c
> b/arch/powerpc/mm/mmu_context_hash64.c
> index 2535828..d0a79f6 100644
> --- a/arch/powerpc/mm/mmu_context_hash64.c
> +++ b/arch/powerpc/mm/mmu_context_hash64.c
> @@ -18,6 +18,7 @@
>  #include <linux/mm.h>
>  #include <linux/spinlock.h>
>  #include <linux/idr.h>
> +#include <linux/percpu.h>
>  #include <linux/module.h>
>  #include <linux/gfp.h>
>  
> @@ -25,6 +26,82 @@
>  
>  static DEFINE_SPINLOCK(mmu_context_lock);
>  static DEFINE_IDA(mmu_context_ida);
> +static DEFINE_IDA(cop_ida);
> +
> +/* Lazy switch the ACOP register */
> +static DEFINE_PER_CPU(unsigned long, acop_reg);
> +
> +void switch_cop(struct mm_struct *next)
> +{
> +	mtspr(SPRN_PID, next->context.pid);
> +	if (next->context.pid &&
> +	    __get_cpu_var(acop_reg) != next->context.acop) {
> +		mtspr(SPRN_ACOP, next->context.acop);
> +		__get_cpu_var(acop_reg) = next->context.acop;
> +	}
> +}

The above doesn't appear to be called anywhere ?

> +int use_cop(unsigned long acop, struct task_struct *task)
> +{
> +	int pid;
> +	int err;
> +	struct mm_struct *mm = get_task_mm(task);
> +
> +	if (!mm)
> +		return -EINVAL;
> +
> +	if (!mm->context.pid) {
> +		if (!ida_pre_get(&cop_ida, GFP_KERNEL))
> +			return -ENOMEM;
> +again:
> +		spin_lock(&mmu_context_lock);
> +		err = ida_get_new_above(&cop_ida, 1, &pid);
> +		spin_unlock(&mmu_context_lock);
> +
> +		if (err == -EAGAIN)
> +			goto again;
> +		else if (err)
> +			return err;
> +
> +		if (pid > HASH64_MAX_PID) {
> +			spin_lock(&mmu_context_lock);
> +			ida_remove(&cop_ida, pid);
> +			spin_unlock(&mmu_context_lock);
> +			return -EBUSY;
> +		}
> +		mm->context.pid = pid;
> +		mtspr(SPRN_PID,  mm->context.pid);

Shouldn't the above be ?

	if (mm == current->active_mm)
		mtspr(....)

> +	}
> +	mm->context.acop |= acop;

Locking ?

> +	get_cpu_var(acop_reg) = mm->context.acop;
> +	mtspr(SPRN_ACOP, mm->context.acop);

Same comment about testing for current.

> +	put_cpu_var(acop_reg);
> +
> +	return mm->context.pid;
> +}
> +EXPORT_SYMBOL(use_cop);
> +
> +void disuse_cop(unsigned long acop, struct mm_struct *mm)
> +{
> +	if (WARN_ON(!mm))
> +		return;
> +
> +	mm->context.acop &= ~acop;

Shouldn't the above need some kind of locking if multiple threads hit it
with different "acop" values ?

> +	if (!mm->context.acop) {
> +		spin_lock(&mmu_context_lock);
> +		ida_remove(&cop_ida, mm->context.pid);
> +		spin_unlock(&mmu_context_lock);
> +		mm->context.pid = 0;
> +		mtspr(SPRN_PID, 0);

Same comment about current.

> +	} else {
> +		get_cpu_var(acop_reg) = mm->context.acop;
> +		mtspr(SPRN_ACOP, mm->context.acop);

Same comment.

> +		put_cpu_var(acop_reg);
> +	}
> +	mmput(mm);
> +}
> +EXPORT_SYMBOL(disuse_cop);
>  
>  /*
>   * The proto-VSID space has 2^35 - 1 segments available for user
> mappings.
> @@ -94,6 +171,8 @@ EXPORT_SYMBOL_GPL(__destroy_context);
>  void destroy_context(struct mm_struct *mm)
>  {
>  	__destroy_context(mm->context.id);
> +	if (mm->context.pid)
> +		ida_remove(&cop_ida, mm->context.pid);
>  	subpage_prot_free(mm);
>  	mm->context.id = NO_CONTEXT;
>  }

Cheers,
Ben.