Re: [PATCH v3 02/10] swiotlb: Factor out slot allocation and free

Lu Baolu
2019-05-02T01:54:44Z

Hi Robin,

On 4/30/19 5:53 PM, Robin Murphy wrote:
> On 30/04/2019 03:02, Lu Baolu wrote:
>> Hi Robin,
>>
>> On 4/29/19 7:06 PM, Robin Murphy wrote:
>>> On 29/04/2019 06:10, Lu Baolu wrote:
>>>> Hi Christoph,
>>>>
>>>> On 4/26/19 11:04 PM, Christoph Hellwig wrote:
>>>>> On Thu, Apr 25, 2019 at 10:07:19AM +0800, Lu Baolu wrote:
>>>>>> This is not VT-d specific. It's just how generic IOMMU works.
>>>>>>
>>>>>> Normally, IOMMU works in paging mode. So if a driver issues DMA with
>>>>>> IOVA  0xAAAA0123, IOMMU can remap it with a physical address
>>>>>> 0xBBBB0123.
>>>>>> But we should never expect IOMMU to remap 0xAAAA0123 with physical
>>>>>> address of 0xBBBB0000. That's the reason why I said that IOMMU
>>>>>> will not
>>>>>> work there.
>>>>>
>>>>> Well, with the iommu it doesn't happen.  With swiotlb it obviosuly
>>>>> can happen, so drivers are fine with it.  Why would that suddenly
>>>>> become an issue when swiotlb is called from the iommu code?
>>>>>
>>>>
>>>> I would say IOMMU is DMA remapping, not DMA engine. :-)
>>>
>>> I'm not sure I really follow the issue here - if we're copying the
>>> buffer to the bounce page(s) there's no conceptual difference from
>>> copying it to SWIOTLB slot(s), so there should be no need to worry
>>> about the original in-page offset.
>>>
>>>  From the reply up-thread I guess you're trying to include an
>>> optimisation to only copy the head and tail of the buffer if it spans
>>> multiple pages, and directly map the ones in the middle, but AFAICS
>>> that's going to tie you to also using strict mode for TLB
>>> maintenance, which may not be a win overall depending on the balance
>>> between invalidation bandwidth vs. memcpy bandwidth. At least if we
>>> use standard SWIOTLB logic to always copy the whole thing, we should
>>> be able to release the bounce pages via the flush queue to allow
>>> 'safe' lazy unmaps.
>>>
>>
>> With respect, even we use the standard SWIOTLB logic, we need to use
>> the strict mode for TLB maintenance.
>>
>> Say, some swiotbl slots are used by untrusted device for bounce page
>> purpose. When the device driver unmaps the IOVA, the slots are freed but
>> the mapping is still cached in IOTLB, hence the untrusted device is
>> still able to access the slots. Then the slots are allocated to other
>> devices. This makes it possible for the untrusted device to access
>> the data buffer of other devices.
>
> Sure, that's indeed how it would work right now - however since the
> bounce pages will be freed and reused by the DMA API layer itself (at
> the same level as the IOVAs) I see no technical reason why we couldn't
> investigate deferred freeing as a future optimisation.

Yes, agreed.

Best regards,
Lu Baolu