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

[Robin Murphy]

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
> > > > > IOVAÂ 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.
> >
> > ÂFrom 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.