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@xxxxxxx> 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@xxxxxxx> writes:
>
> > 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
