Messages in this thread | | | Subject | Re: [PATCH v3 02/10] swiotlb: Factor out slot allocation and free | From | Robin Murphy <> | Date | Tue, 30 Apr 2019 10:53:48 +0100 |
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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.
Robin.
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