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From: "Sven Peter" <sven@svenpeter.dev>
To: "Robin Murphy" <robin.murphy@arm.com>,
	"Sven Peter" <iommu@lists.linux-foundation.org>
Cc: "Arnd Bergmann" <arnd@kernel.org>,
	"Will Deacon" <will@kernel.org>,
	"Hector Martin" <marcan@marcan.st>,
	linux-kernel@vger.kernel.org, "Alexander Graf" <graf@amazon.com>,
	"Mohamed Mediouni" <mohamed.mediouni@caramail.com>
Subject: Re: [RFC PATCH 2/3] iommu/dma-iommu: Support iovad->granule > PAGE_SIZE
Date: Sat, 07 Aug 2021 13:47:21 +0200	[thread overview]
Message-ID: <dadbd8b0-171a-4008-8a2e-f68abfed9285@www.fastmail.com> (raw)
In-Reply-To: <d289a555-9e3b-b948-1883-2ee4c915da4c@arm.com>



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

  parent reply	other threads:[~2021-08-07 11:48 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-06 15:55 [RFC PATCH 0/3] iommu/dma-iommu: Support IOMMU page size larger than the CPU page size Sven Peter
2021-08-06 15:55 ` [RFC PATCH 1/3] iommu: Move IOMMU pagesize check to attach_device Sven Peter
2021-08-06 15:55 ` [RFC PATCH 2/3] iommu/dma-iommu: Support iovad->granule > PAGE_SIZE Sven Peter
2021-08-06 18:04   ` Robin Murphy
2021-08-07  8:41     ` Sven Peter
2021-08-09 18:37       ` Robin Murphy
2021-08-09 19:57         ` Sven Peter
2021-08-07 11:47     ` Sven Peter [this message]
2021-08-09 17:41       ` Robin Murphy
2021-08-09 20:45         ` Sven Peter
2021-08-10  9:51           ` Robin Murphy
2021-08-11 20:18             ` Sven Peter
2021-08-12 12:43               ` Robin Murphy
2021-08-06 15:55 ` [RFC PATCH 3/3] iommu: Introduce __IOMMU_DOMAIN_LARGE_PAGES Sven Peter

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