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Date: Tue, 10 Aug 2021 10:51:52 +0100 From: Robin Murphy <robin.murphy@....com> To: Sven Peter <sven@...npeter.dev>, 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 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 >
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