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Message-ID: <87il8wr348.fsf@meer.lwn.net>
Date: Wed, 30 Aug 2023 08:55:51 -0600
From: Jonathan Corbet <corbet@....net>
To: Petr Tesarik <petrtesarik@...weicloud.com>,
Stefano Stabellini <sstabellini@...nel.org>,
Russell King <linux@...linux.org.uk>,
Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
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"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
"H. Peter Anvin" <hpa@...or.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
"Rafael J. Wysocki" <rafael@...nel.org>,
Juergen Gross <jgross@...e.com>,
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Vlastimil Babka <vbabka@...e.cz>,
Roman Gushchin <roman.gushchin@...ux.dev>,
Hyeonggon Yoo <42.hyeyoo@...il.com>,
Petr Tesarik <petr.tesarik.ext@...wei.com>,
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Cc: Roberto Sassu <roberto.sassu@...weicloud.com>, petr@...arici.cz
Subject: Re: [PATCH v7 9/9] swiotlb: search the software IO TLB only if the
device makes use of it
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...
Thanks,
jon
Jonathan Corbet <corbet@....net> writes:
> Petr Tesarik <petrtesarik@...weicloud.com> writes:
>
>> From: Petr Tesarik <petr.tesarik.ext@...wei.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@...wei.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
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