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

["Petr Tesařík" <petr@tesarici.cz>]

Hi all,

sorry for my late reply; I've been away from my work setup for a
month...

On Wed, 30 Aug 2023 08:55:51 -0600
Jonathan Corbet <corbet@lwn.net> wrote:

> 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...

Yes, I believe the check was originally in is_swiotlb_buffer(), but it
got lost during one of the numerous rebases of this patch set. Let me
send a follow-up patch after making sure it actually works.

Petr T

> 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