From: Petr Tesarik <[email protected]>
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 <[email protected]>
---
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;
#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;
#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();
+
*retpool = pool;
return index;
}
--
2.25.1
Petr Tesarik <[email protected]> writes:
> From: Petr Tesarik <[email protected]>
>
> 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 <[email protected]>
> ---
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
On Wed, Aug 09, 2023 at 03:20:43PM -0600, Jonathan Corbet wrote:
> > 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?
It's very much unused. Petr, I guess you wanted to use this in
is_swiotlb_buffer to avoid the lookup unless required. Can you send
a follow up?