Re: [RFC PATCH 2/3] iommu/dma-iommu: Support iovad->granule > PAGE_SIZE

[Robin Murphy]

On 2021-08-09 21:45, Sven Peter wrote:
> On Mon, Aug 9, 2021, at 19:41, Robin Murphy wrote:
> > On 2021-08-07 12:47, Sven Peter via iommu wrote:
> > > On Fri, Aug 6, 2021, at 20:04, Robin Murphy wrote:
> > > > On 2021-08-06 16:55, Sven Peter via iommu wrote:
> > > > > @@ -1006,6 +1019,31 @@ static int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg,
> > > > >     	if (dev_is_untrusted(dev))
> > > > >     		return iommu_dma_map_sg_swiotlb(dev, sg, nents, dir, attrs);
> > > > >     
> > > > > +	/*
> > > > > +	 * If the IOMMU pagesize is larger than the CPU pagesize we will
> > > > > +	 * very likely run into sgs with a physical address that is not aligned
> > > > > +	 * to an IOMMU page boundary. Fall back to just mapping every entry
> > > > > +	 * independently with __iommu_dma_map then.
> > > >
> > > > Scatterlist segments often don't have nicely aligned ends, which is why
> > > > we already align things to IOVA granules in main loop here. I think in
> > > > principle we'd just need to move the non-IOVA-aligned part of the
> > > > address from sg->page to sg->offset in the temporary transformation for
> > > > the rest of the assumptions to hold. I don't blame you for being timid
> > > > about touching that, though - it took me 3 tries to get right when I
> > > > first wrote it...
> > >
> > >
> > > I've spent some time with that code now and I think we cannot use it
> > > but have to fall back to iommu_dma_map_sg_swiotlb (even though that swiotlb
> > > part is a lie then):
> > >
> > > When we have sg_phys(s) = 0x802e65000 with s->offset = 0 the paddr
> > > is aligned to PAGE_SIZE but has an offset of 0x1000 from something
> > > the IOMMU can map.
> > > Now this would result in s->offset = -0x1000 which is already weird
> > > enough.
> > > Offset is unsigned (and 32bit) so this will actually look like
> > > s->offset = 0xfffff000 then, which isn't much better.
> > > And then sg_phys(s) = 0x902e64000 (instead of 0x802e64000) and
> > > we'll map some random memory in iommu_map_sg_atomic and a little bit later
> > > everything explodes.
> > >
> > > Now I could probably adjust the phys addr backwards and make sure offset is
> > > always positive (and possibly larger than PAGE_SIZE) and later restore it
> > > in __finalise_sg then but I feel like that's pushing this a little bit too far.
> >
> > Yes, that's what I meant. At a quick guess, something like the
> > completely untested diff below.
>
> That unfortunately results in unaligned mappings

You mean it even compiles!? :D

> [    9.630334] iommu: unaligned: iova 0xbff40000 pa 0x0000000801a3b000 size 0x4000 min_pagesz 0x4000
>
> I'll take a closer look later this week and see if I can fix it.

On reflection, "s->offset ^ s_iova_off" is definitely wrong, that more 
likely wants to be "s->offset & ~s_iova_off".

Robin.

> > It really comes down to what we want to
> > achieve here - if it's just to make this thing work at all, then I'd
> > favour bolting on the absolute minimum changes, possibly even cheating
> > by tainting the kernel and saying all bets are off instead of trying to
> > handle the more involved corners really properly. However if you want to
> > work towards this being a properly-supported thing, then I think it's
> > worth generalising the existing assumptions of page alignment from the
> > beginning.
>
> I'd like to try and see if we can make this a properly-supported thing.
>
> That will likely take a few iterations but realistically the rest of the drivers
> required to make this platform actually useful (and especially the display controller
> and GPU drivers) won't be ready for a few more months anyway. And even on 4KB PAGE_SIZE
> kernels half the USB ports and NVMe will work fine, which should be enough to install
> a distro and some third-party package that just ships the distro kernel with 16KB
> pages.
>
>
>
>
> Sven