Re: [PATCH] p54: avoid accessing the data mapped to streaming DMA

[Kalle Valo <kvalo@codeaurora.org>]

Christian Lamparter <chunkeey@gmail.com> writes:

> On 2020-08-02 15:29, Jia-Ju Bai wrote:
>> In p54p_tx(), skb->data is mapped to streaming DMA on line 337:
>>    mapping = pci_map_single(..., skb->data, ...);
>>
>> Then skb->data is accessed on line 349:
>>    desc->device_addr = ((struct p54_hdr *)skb->data)->req_id;
>>
>> This access may cause data inconsistency between CPU cache and hardware.
>>
>> To fix this problem, ((struct p54_hdr *)skb->data)->req_id is stored in
>> a local variable before DMA mapping, and then the driver accesses this
>> local variable instead of skb->data.
>
> Interesting. Please bear with me here. From my understanding, the
> streaming direction is set to PCI_DMA_TODEVICE. So is it really
> possible for the hardware to interfere with the data without the IOMMU
> catching this?

Also is there any documentation about this scenario? I would like to
understand this better.

> (That said, patch looks be fine. I'll need to dust off a old PCI PC to
> check this with real hardware, if requested.)

No need to test with real hardware for my sake, but a careful review is
very much appreciated.

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