linux-kernel - Re: [PATCH v3 2/5] dma-iommu: fix arch_sync

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Date:   Thu, 12 Aug 2021 11:38:02 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     David Stevens <stevensd@...omium.org>
Cc:     Will Deacon <will@...nel.org>, Joerg Roedel <joro@...tes.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Tom Murphy <murphyt7@....ie>, iommu@...ts.linux-foundation.org,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/5] dma-iommu: fix arch_sync_dma for map

On 2021-08-12 10:21, David Stevens wrote:
> On Thu, Aug 12, 2021 at 3:47 AM Robin Murphy <robin.murphy@....com> wrote:
>>
>> On 2021-08-11 03:42, David Stevens wrote:
>>> From: David Stevens <stevensd@...omium.org>
>>>
>>> When calling arch_sync_dma, we need to pass it the memory that's
>>> actually being used for dma. When using swiotlb bounce buffers, this is
>>> the bounce buffer. Move arch_sync_dma into the __iommu_dma_map_swiotlb
>>> helper, so it can use the bounce buffer address if necessary. This also
>>> means it is no longer necessary to call iommu_dma_sync_sg_for_device in
>>> iommu_dma_map_sg for untrusted devices.
>>>
>>> Fixes: 82612d66d51d ("iommu: Allow the dma-iommu api to use bounce buffers")
>>> Signed-off-by: David Stevens <stevensd@...omium.org>
>>> ---
>>>    drivers/iommu/dma-iommu.c | 16 +++++++---------
>>>    1 file changed, 7 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
>>> index 54e103b989d9..4f0cc4a0a61f 100644
>>> --- a/drivers/iommu/dma-iommu.c
>>> +++ b/drivers/iommu/dma-iommu.c
>>> @@ -576,6 +576,9 @@ static dma_addr_t __iommu_dma_map_swiotlb(struct device *dev, phys_addr_t phys,
>>>                memset(padding_start, 0, padding_size);
>>>        }
>>>
>>> +     if (!coherent && !(attrs & DMA_ATTR_SKIP_CPU_SYNC))
>>
>> Make that an "else if" - otherwise you're just reintroducing the same
>> thing that the third hunk is trying to clean up.
> 
> swiotlb_tbl_map_single only copies into the swiotlb buffer, it doesn't
> do any architectural syncing. So this block is necessary in the
> swiotlb case as well, for iommu_dma_map_page to work properly.

Urgh, apologies again - I had a mental short-circuit and was convinced 
that the SWIOTLB call already did its own cache maintenance. I really 
should have learned by now not to review fiddly patches like these while 
tired...
> The third chunk is a separate optimization, so I'll split it out into
> its own patch.

It's still a logical part of this change - cleaning up the potentially 
wrong sync while you make sure the right one happens. Even more so if 
you make the fix the better way by having iommu_dma_map_sg_swiotlb() 
call iommu_dma_map_page().

Robin.