Re: [PATCH v2] arm64: do not set dma masks that device connection can't handle

[Arnd Bergmann]

On Tuesday, January 10, 2017 1:25:12 PM CET Robin Murphy wrote:
> On 10/01/17 12:47, Nikita Yushchenko wrote:
> >> The point here is that an IOMMU doesn't solve your issue, and the
> >> IOMMU-backed DMA ops need the same treatment. In light of that, it really
> >> feels to me like the DMA masks should be restricted in of_dma_configure
> >> so that the parent mask is taken into account there, rather than hook
> >> into each set of DMA ops to intercept set_dma_mask. We'd still need to
> >> do something to stop dma_set_mask widening the mask if it was restricted
> >> by of_dma_configure, but I think Robin (cc'd) was playing with that.
> > 
> > What issue "IOMMU doesn't solve"?
> > 
> > Issue I'm trying to address is - inconsistency within swiotlb
> > dma_map_ops, where (1) any wide mask is silently accepted, but (2) then
> > mask is used to decide if bounce buffers are needed or not. This
> > inconsistency causes NVMe+R-Car cobmo not working (and breaking memory
> > instead).
> 
> The fundamental underlying problem is the "any wide mask is silently
> accepted" part, and that applies equally to IOMMU ops as well.

It's a much rarer problem for the IOMMU case though, because it only
impacts devices that are restricted to addressing of less than 32-bits.

If you have an IOMMU enabled, the dma-mapping interface does not care
if the device can do wider than 32 bit addressing, as it will never
hand out IOVAs above 0xffffffff.

> > I just can't think out what similar issue iommu can have.
> > Do you mean that in iommu case, mask also must not be set to whatever
> > wider than initial value? Why? What is the use of mask in iommu case? Is
> > there any real case when iommu can't address all memory existing in the
> > system?
> 
> There's a very subtle misunderstanding there - the DMA mask does not
> describe the memory a device can address, it describes the range of
> addresses the device is capable of generating. Yes, in the non-IOMMU
> case they are equivalent, but once you put an IOMMU in between, the
> problem is merely shifted from "what range of physical addresses can
> this device access" to "what range of IOVAs is valid to give to this
> device" - the fact that those IOVAs can map to any underlying physical
> address only obviates the need for any bouncing at the memory end; it
> doesn't remove the fact that the device has a hardware addressing
> limitation which needs to be accommodated.
> 
> The thread Will linked to describes that equivalent version of your
> problem - the IOMMU gives the device 48-bit addresses which get
> erroneously truncated because it doesn't know that only 42 bits are
> actually wired up. That situation still requires the device's DMA mask
> to correctly describe its addressing capability just as yours does.

That problem should only impact virtual machines which have a guest
bus address space covering more than 42 bits of physical RAM, whereas
the problem we have with swiotlb is for the dma-mapping interface.

> > With this direction, semantics of dma mask becomes even more
> > questionable. I'd say dma_mask is candidate for removal (or to move to
> > swiotlb's or iommu's local area)
> 
> We still need a way for drivers to communicate a device's probed
> addressing capability to SWIOTLB, so there's always going to have to be
> *some* sort of public interface. Personally, the change in semantics I'd
> like to see is to make dma_set_mask() only fail if DMA is entirely
> disallowed - in the normal case it would always succeed, but the DMA API
> implementation would be permitted to set a smaller mask than requested
> (this is effectively what the x86 IOMMU ops do already).

With swiotlb enabled, it only needs to fail if the mask does not contain
the swiotlb bounce buffer area, either because the start of RAM is outside
of the mask, or the bounce area has been allocated at the end of ZONE_DMA
and the mask is smaller than ZONE_DMA.

	Arnd