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From:   Jonathan Corbet <corbet@lwn.net>
To:     Petr Tesarik <petrtesarik@huaweicloud.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        Russell King <linux@armlinux.org.uk>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
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        Vlastimil Babka <vbabka@suse.cz>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>,
        Petr Tesarik <petr.tesarik.ext@huawei.com>,
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Cc:     Roberto Sassu <roberto.sassu@huaweicloud.com>, petr@tesarici.cz
Subject: Re: [PATCH v7 9/9] swiotlb: search the software IO TLB only if the
 device makes use of it
In-Reply-To: <adea71bd1fa8660d4c3157a562431ad8127016d4.1690871004.git.petr.tesarik.ext@huawei.com>
References: <cover.1690871004.git.petr.tesarik.ext@huawei.com>
 <adea71bd1fa8660d4c3157a562431ad8127016d4.1690871004.git.petr.tesarik.ext@huawei.com>
Date:   Wed, 09 Aug 2023 15:20:43 -0600
Message-ID: <87a5uz3ob8.fsf@meer.lwn.net>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

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