Message-Id: <7b6cf00c-86ea-43c4-861f-f1cc32d5852d@www.fastmail.com>
Date:   Wed, 11 Aug 2021 22:18:32 +0200
From:   "Sven Peter" <sven@...npeter.dev>
To:     "Robin Murphy" <robin.murphy@....com>,
        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 Tue, Aug 10, 2021, at 11:51, Robin Murphy wrote:
> 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

I was more impressed that it already almost worked correctly :)

> 
> > [    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.
> 


If I change

		sg_set_page(s, phys_to_page(sg_phys(s)), s_length,
			    s_iova_off & ~PAGE_MASK);

in __finalise_sg (and the same thing in __invalidate_sg) to

		sg_set_page(s, phys_to_page(sg_phys(s) + s_iova_off), s_length,
			    s_iova_off & ~PAGE_MASK);

then it also restores the original fields correctly.


What is the proper way to credit you for coming up with this?
Do you create the commit and I apply it to my local tree and
include it in my submission once I have fixed the other
issues? Or do I create the commit and put a Suggested-by
in the message?


Either way, here's the patch that I have right now:

diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
index 7ce74476699d..ba31dc59566d 100644
--- a/drivers/iommu/dma-iommu.c
+++ b/drivers/iommu/dma-iommu.c
@@ -907,8 +907,8 @@ static int __finalise_sg(struct device *dev, struct scatterlist *sg, int nents,
 		unsigned int s_length = sg_dma_len(s);
 		unsigned int s_iova_len = s->length;

-		s->offset += s_iova_off;
-		s->length = s_length;
+		sg_set_page(s, phys_to_page(sg_phys(s) + s_iova_off), s_length,
+			    s_iova_off & ~PAGE_MASK);
 		sg_dma_address(s) = DMA_MAPPING_ERROR;
 		sg_dma_len(s) = 0;

@@ -952,10 +952,11 @@ static void __invalidate_sg(struct scatterlist *sg, int nents)
 	int i;

 	for_each_sg(sg, s, nents, i) {
-		if (sg_dma_address(s) != DMA_MAPPING_ERROR)
-			s->offset += sg_dma_address(s);
 		if (sg_dma_len(s))
-			s->length = sg_dma_len(s);
+			sg_set_page(s,
+				    phys_to_page(sg_phys(s) + sg_dma_address(s)),
+				    sg_dma_len(s),
+				    sg_dma_address(s) & ~PAGE_MASK);
 		sg_dma_address(s) = DMA_MAPPING_ERROR;
 		sg_dma_len(s) = 0;
 	}
@@ -1031,15 +1032,16 @@ static int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg,
 	 * stashing the unaligned parts in the as-yet-unused DMA fields.
 	 */
 	for_each_sg(sg, s, nents, i) {
-		size_t s_iova_off = iova_offset(iovad, s->offset);
+		phys_addr_t s_phys = sg_phys(s);
+		size_t s_iova_off = iova_offset(iovad, s_phys);
 		size_t s_length = s->length;
 		size_t pad_len = (mask - iova_len + 1) & mask;

 		sg_dma_address(s) = s_iova_off;
 		sg_dma_len(s) = s_length;
-		s->offset -= s_iova_off;
 		s_length = iova_align(iovad, s_length + s_iova_off);
-		s->length = s_length;
+		sg_set_page(s, phys_to_page(s_phys - s_iova_off),
+			    s_length, s->offset & ~s_iova_off);

 		/*
 		 * Due to the alignment of our single IOVA allocation, we can





Sven

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