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

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Message-ID: <a721f8e3-4c1d-afb2-3ae2-eb1360e1eaca@arm.com>
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
>