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

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Message-Id: <cf78f795-1e75-45c9-a759-018f17cfaed9@www.fastmail.com>
Date:   Mon, 09 Aug 2021 22:45:15 +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>,
        "Hector Martin" <marcan@...can.st>, linux-kernel@...r.kernel.org,
        "Alexander Graf" <graf@...zon.com>,
        "Mohamed Mediouni" <mohamed.mediouni@...amail.com>,
        "Will Deacon" <will@...nel.org>
Subject: Re: [RFC PATCH 2/3] iommu/dma-iommu: Support iovad->granule > PAGE_SIZE



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

[    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.

> 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