Re: [RFC PATCH 2/3] iommu/dma-iommu: Support iovad->granule > PAGE_SIZE
From: Robin Murphy
Date: Thu Aug 12 2021 - 08:44:04 EST
On 2021-08-11 21:18, Sven Peter wrote:
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.
Ah, good point, once again this proves to be right on the limit of how
many moving parts I can hold in my head without working it through on
paper (or in a debugger). FWIW my thought there was that sg_phys(s)
would be enough to "re-normalise" things from a state where s->page_link
was rounded down and s->offset held the PAGE_SIZE multiple (which is
what the XOR did do), but of course that wasn't right to begin with. In
fact, s->offset must always be negligible when restoring so you could
arguably open-code page_to_phys(sg_page(s)), but that might be a bit too
much of a mouthful for a theoretical micro-optimisation (note that
trying to avoid the phys_addr_t round-trip by offsetting the page
pointer itself might appear to work for SPARSEMEM_VMEMMAP, but I'm
pretty sure it's not valid in general).
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?
Suggested-by is fine by me, but if you feel there's enough of my diff
left that you haven't had to put right then you're also welcome to have
these instead:
Co-developed-by: Robin Murphy <robin.murphy@xxxxxxx>
Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
Cheers,
Robin.
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