Re: [PATCH 01/12] dma-buf: add dynamic caching of sg_table

["Christian König" <ckoenig.leichtzumerken@gmail.com>]

Am 22.05.19 um 18:17 schrieb Sumit Semwal:
> Hi Christian,
>
> On Sat, 27 Apr 2019 at 05:31, Liam Mark <lmark@codeaurora.org> wrote:
>> On Tue, 16 Apr 2019, Christian König wrote:
>>
>>> To allow a smooth transition from pinning buffer objects to dynamic
>>> invalidation we first start to cache the sg_table for an attachment
>>> unless the driver explicitly says to not do so.
>>>
>>> ---
>>>   drivers/dma-buf/dma-buf.c | 24 ++++++++++++++++++++++++
>>>   include/linux/dma-buf.h   | 11 +++++++++++
>>>   2 files changed, 35 insertions(+)
>>>
>>> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
>>> index 7c858020d14b..65161a82d4d5 100644
>>> --- a/drivers/dma-buf/dma-buf.c
>>> +++ b/drivers/dma-buf/dma-buf.c
>>> @@ -573,6 +573,20 @@ struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
>>>        list_add(&attach->node, &dmabuf->attachments);
>>>
>>>        mutex_unlock(&dmabuf->lock);
>>> +
>>> +     if (!dmabuf->ops->dynamic_sgt_mapping) {
>>> +             struct sg_table *sgt;
>>> +
>>> +             sgt = dmabuf->ops->map_dma_buf(attach, DMA_BIDIRECTIONAL);
>>> +             if (!sgt)
>>> +                     sgt = ERR_PTR(-ENOMEM);
>>> +             if (IS_ERR(sgt)) {
>>> +                     dma_buf_detach(dmabuf, attach);
>>> +                     return ERR_CAST(sgt);
>>> +             }
>>> +             attach->sgt = sgt;
>>> +     }
>>> +
>>>        return attach;
>>>
>>>   err_attach:
>>> @@ -595,6 +609,10 @@ void dma_buf_detach(struct dma_buf *dmabuf, struct dma_buf_attachment *attach)
>>>        if (WARN_ON(!dmabuf || !attach))
>>>                return;
>>>
>>> +     if (attach->sgt)
>>> +             dmabuf->ops->unmap_dma_buf(attach, attach->sgt,
>>> +                                        DMA_BIDIRECTIONAL);
>>> +
>>>        mutex_lock(&dmabuf->lock);
>>>        list_del(&attach->node);
>>>        if (dmabuf->ops->detach)
>>> @@ -630,6 +648,9 @@ struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *attach,
>>>        if (WARN_ON(!attach || !attach->dmabuf))
>>>                return ERR_PTR(-EINVAL);
>>>
>>> +     if (attach->sgt)
>>> +             return attach->sgt;
>>> +
>> I am concerned by this change to make caching the sg_table the default
>> behavior as this will result in the exporter's map_dma_buf/unmap_dma_buf
>> calls are no longer being called in
>> dma_buf_map_attachment/dma_buf_unmap_attachment.
> Probably this concern from Liam got lost between versions of your
> patches; could we please request a reply to these points here?

Sorry I indeed never got this mail, but this is actually not an issue 
because Daniel had similar concerns and we didn't made this the default 
in the final version.

>> This seems concerning to me as it appears to ignore the cache maintenance
>> aspect of the map_dma_buf/unmap_dma_buf calls.
>> For example won't this potentially cause issues for clients of ION.
>>
>> If we had the following
>> - #1 dma_buf_attach coherent_device
>> - #2 dma_buf attach non_coherent_device
>> - #3 dma_buf_map_attachment non_coherent_device
>> - #4 non_coherent_device writes to buffer
>> - #5 dma_buf_unmap_attachment non_coherent_device
>> - #6 dma_buf_map_attachment coherent_device
>> - #7 coherent_device reads buffer
>> - #8 dma_buf_unmap_attachment coherent_device
>>
>> There wouldn't be any CMO at step #5 anymore (specifically no invalidate)
>> so now at step #7 the coherent_device could read a stale cache line.
>>
>> Also, now by default dma_buf_unmap_attachment no longer removes the
>> mappings from the iommu, so now by default dma_buf_unmap_attachment is not
>> doing what I would expect and clients are losing the potential sandboxing
>> benefits of removing the mappings.
>> Shouldn't this caching behavior be something that clients opt into instead
>> of being the default?

Well, it seems you are making incorrect assumptions about the cache 
maintenance of DMA-buf here.

At least for all DRM devices I'm aware of mapping/unmapping an 
attachment does *NOT* have any cache maintenance implications.

E.g. the use case you describe above would certainly fail with amdgpu, 
radeon, nouveau and i915 because mapping a DMA-buf doesn't stop the 
exporter from reading/writing to that buffer (just the opposite actually).

All of them assume perfectly coherent access to the underlying memory. 
As far as I know there is no documented cache maintenance requirements 
for DMA-buf.

The IOMMU concern on the other hand is certainly valid and I perfectly 
agree that keeping the mapping time as short as possible is desirable.

Regards,
Christian.

>> Liam
>>
>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
>> a Linux Foundation Collaborative Project
>>
> Best,
> Sumit.

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