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

[Jonathan Corbet <corbet@xxxxxxx>]

Petr Tesarik <petrtesarik@xxxxxxxxxxxxxxx> writes:

> From: Petr Tesarik <petr.tesarik.ext@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> ---

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