Re: [PATCH v7 9/9] swiotlb: search the software IO TLB only if the device makes use of it

[Jonathan Corbet <corbet@lwn.net>]

So it seems this code got merged without this question ever being
answered.  Sorry if it's a dumb one, but I don't think this
functionality works as advertised...

Thanks,

jon

Jonathan Corbet <corbet@lwn.net> writes:

> Petr Tesarik <petrtesarik@huaweicloud.com> writes:
>
>> From: Petr Tesarik <petr.tesarik.ext@huawei.com>
>>
>> Skip searching the software IO TLB if a device has never used it, making
>> sure these devices are not affected by the introduction of multiple IO TLB
>> memory pools.
>>
>> Additional memory barrier is required to ensure that the new value of the
>> flag is visible to other CPUs after mapping a new bounce buffer. For
>> efficiency, the flag check should be inlined, and then the memory barrier
>> must be moved to is_swiotlb_buffer(). However, it can replace the existing
>> barrier in swiotlb_find_pool(), because all callers use is_swiotlb_buffer()
>> first to verify that the buffer address belongs to the software IO TLB.
>>
>> Signed-off-by: Petr Tesarik <petr.tesarik.ext@huawei.com>
>> ---
>
> Excuse me if this is a silly question, but I'm not able to figure it out
> on my own...
>
>>  include/linux/device.h  |  2 ++
>>  include/linux/swiotlb.h |  7 ++++++-
>>  kernel/dma/swiotlb.c    | 14 ++++++--------
>>  3 files changed, 14 insertions(+), 9 deletions(-)
>>
>> diff --git a/include/linux/device.h b/include/linux/device.h
>> index 5fd89c9d005c..6fc808d22bfd 100644
>> --- a/include/linux/device.h
>> +++ b/include/linux/device.h
>> @@ -628,6 +628,7 @@ struct device_physical_location {
>>   * @dma_io_tlb_mem: Software IO TLB allocator.  Not for driver use.
>>   * @dma_io_tlb_pools:	List of transient swiotlb memory pools.
>>   * @dma_io_tlb_lock:	Protects changes to the list of active pools.
>> + * @dma_uses_io_tlb: %true if device has used the software IO TLB.
>>   * @archdata:	For arch-specific additions.
>>   * @of_node:	Associated device tree node.
>>   * @fwnode:	Associated device node supplied by platform firmware.
>> @@ -737,6 +738,7 @@ struct device {
>>  #ifdef CONFIG_SWIOTLB_DYNAMIC
>>  	struct list_head dma_io_tlb_pools;
>>  	spinlock_t dma_io_tlb_lock;
>> +	bool dma_uses_io_tlb;
>
> You add this new member here, fine...
>
>>  #endif
>>  	/* arch specific additions */
>>  	struct dev_archdata	archdata;
>> diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
>> index 8371c92a0271..b4536626f8ff 100644
>> --- a/include/linux/swiotlb.h
>> +++ b/include/linux/swiotlb.h
>> @@ -172,8 +172,13 @@ static inline bool is_swiotlb_buffer(struct device *dev, phys_addr_t paddr)
>>  	if (!mem)
>>  		return false;
>>  
>> -	if (IS_ENABLED(CONFIG_SWIOTLB_DYNAMIC))
>> +	if (IS_ENABLED(CONFIG_SWIOTLB_DYNAMIC)) {
>> +		/* Pairs with smp_wmb() in swiotlb_find_slots() and
>> +		 * swiotlb_dyn_alloc(), which modify the RCU lists.
>> +		 */
>> +		smp_rmb();
>>  		return swiotlb_find_pool(dev, paddr);
>> +	}
>>  	return paddr >= mem->defpool.start && paddr < mem->defpool.end;
>>  }
>>  
>> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
>> index adf80dec42d7..d7eac84f975b 100644
>> --- a/kernel/dma/swiotlb.c
>> +++ b/kernel/dma/swiotlb.c
>> @@ -730,7 +730,7 @@ static void swiotlb_dyn_alloc(struct work_struct *work)
>>  
>>  	add_mem_pool(mem, pool);
>>  
>> -	/* Pairs with smp_rmb() in swiotlb_find_pool(). */
>> +	/* Pairs with smp_rmb() in is_swiotlb_buffer(). */
>>  	smp_wmb();
>>  }
>>  
>> @@ -764,11 +764,6 @@ struct io_tlb_pool *swiotlb_find_pool(struct device *dev, phys_addr_t paddr)
>>  	struct io_tlb_mem *mem = dev->dma_io_tlb_mem;
>>  	struct io_tlb_pool *pool;
>>  
>> -	/* Pairs with smp_wmb() in swiotlb_find_slots() and
>> -	 * swiotlb_dyn_alloc(), which modify the RCU lists.
>> -	 */
>> -	smp_rmb();
>> -
>>  	rcu_read_lock();
>>  	list_for_each_entry_rcu(pool, &mem->pools, node) {
>>  		if (paddr >= pool->start && paddr < pool->end)
>> @@ -813,6 +808,7 @@ void swiotlb_dev_init(struct device *dev)
>>  #ifdef CONFIG_SWIOTLB_DYNAMIC
>>  	INIT_LIST_HEAD(&dev->dma_io_tlb_pools);
>>  	spin_lock_init(&dev->dma_io_tlb_lock);
>> +	dev->dma_uses_io_tlb = false;
>
> ...here you initialize it, fine...
>
>>  #endif
>>  }
>>  
>> @@ -1157,9 +1153,11 @@ static int swiotlb_find_slots(struct device *dev, phys_addr_t orig_addr,
>>  	list_add_rcu(&pool->node, &dev->dma_io_tlb_pools);
>>  	spin_unlock_irqrestore(&dev->dma_io_tlb_lock, flags);
>>  
>> -	/* Pairs with smp_rmb() in swiotlb_find_pool(). */
>> -	smp_wmb();
>>  found:
>> +	dev->dma_uses_io_tlb = true;
>> +	/* Pairs with smp_rmb() in is_swiotlb_buffer() */
>> +	smp_wmb();
>> +
>
> ...and here you set it if swiotlb is used.
>
> But, as far as I can tell, you don't actually *use* this field anywhere.
> What am I missing?
>
> Thanks,
>
> jon