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

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Message-Id: <dadbd8b0-171a-4008-8a2e-f68abfed9285@www.fastmail.com>
Date:   Sat, 07 Aug 2021 13:47:21 +0200
From:   "Sven Peter" <sven@...npeter.dev>
To:     "Robin Murphy" <robin.murphy@....com>,
        "Sven Peter" <iommu@...ts.linux-foundation.org>
Cc:     "Arnd Bergmann" <arnd@...nel.org>, "Will Deacon" <will@...nel.org>,
        "Hector Martin" <marcan@...can.st>, linux-kernel@...r.kernel.org,
        "Alexander Graf" <graf@...zon.com>,
        "Mohamed Mediouni" <mohamed.mediouni@...amail.com>
Subject: Re: [RFC PATCH 2/3] iommu/dma-iommu: Support iovad->granule > PAGE_SIZE



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.



Sven