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

[Kalle Valo <kvalo@codeaurora.org>]

Christian Lamparter <chunkeey@gmail.com> writes:

> On 2020-08-26 18:02, Kalle Valo wrote:
>> 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.
>
> I usually rely on the information present in Documentation:
> <https://www.kernel.org/doc/Documentation/DMA-API-HOWTO.txt>
>
> The relevant extract for p54's DMA_TO_DEVICE decision likely comes from:
>
> "For Networking drivers, it's a rather simple affair.  For transmit
> packets, map/unmap them with the DMA_TO_DEVICE direction
> specifier.  For receive packets, just the opposite, map/unmap them
> with the DMA_FROM_DEVICE direction specifier."
>
> "Only streaming mappings specify a direction, consistent mappings
> implicitly have a direction attribute setting of DMA_BIDIRECTIONAL."

This is not very clearly written, I guess it's assumed everyone know
this stuff :)

> But looking around on the Internet, I came across this in "Chapter 15.
> Memory Mapping and DMA" of the Linux Device Drivers, 3rd Edition:
>
> <https://www.oreilly.com/library/view/linux-device-drivers/0596005903/ch15.html>
>
> |Setting up streaming DMA mappings
> |[...]
> |
> |Some important rules apply to streaming DMA mappings:
> | * [...]
> |
> | * Once a buffer has been mapped, it belongs to the device, not the
> |   processor. Until the buffer has been unmapped, the driver should
> not |   touch its contents in any way. Only after dma_unmap_single has
> been |   called is it safe for the driver to access the contents of
> the
> |   buffer (with one exception that we see shortly). Among other things,
> |   this rule implies that a buffer being written to a device cannot be
> |   mapped until it contains all the data to write."
> |
> | [...] (More informative text, but only)
>
> From the sentence "Once a buffer has been mapped, it belongs to the
> device, not the processor". I think that Jia-Ju Bai's patch is doing
> exactly this "by the book".

Yeah, this is much better and understandable. Thanks for checking.

> Therefore, it should be applied and backported:
>
> Cc: <stable@vger.kernel.org>

Ok, I'll add that.

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