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Date: Fri, 29 Mar 2024 15:18:16 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Dominique Martinet <dominique.martinet@...ark-techno.com>
CC: "hch@....de" <hch@....de>, "m.szyprowski@...sung.com"
	<m.szyprowski@...sung.com>, "robin.murphy@....com" <robin.murphy@....com>,
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	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"will@...nel.org" <will@...nel.org>, "petr@...arici.cz" <petr@...arici.cz>,
	"roberto.sassu@...weicloud.com" <roberto.sassu@...weicloud.com>,
	"lukas@...mn.com" <lukas@...mn.com>
Subject: RE: [PATCH 1/1] swiotlb: Fix swiotlb_bounce() to do partial sync's
 correctly

From: Dominique Martinet <dominique.martinet@...ark-techno.com> Sent: Thursday, March 28, 2024 10:09 PM
> 
> Dominique Martinet wrote on Wed, Mar 27, 2024 at 03:05:18PM +0900:
> > Unfortunately that was ages ago so I don't really remember exactly on
> > which device that was reproduced.. Given the Cc I'm sure Lukas had hit
> > it on the MNT reform (i.MX8MQ), but I did say I tested it so I probably
> > could reproduce on my i.MX8MP?
> > I'll try to give a try at reproducing the old bug, and if I do test your
> > fix over next week.
> 
> grmbl, sorry I cannot reproduce the problem on devices I have readily
> accessible, and don't have time to dig out my reform to test there in
> the forseeable future, so cannot confirm if that also fixes the problem
> we reported two years ago.
>
> However I had misunderstood your patch, I thought you were also
> reverting commit 5f89468e2f06 ("swiotlb: manipulate orig_addr when
> tlb_addr has offset") but you're keeping it and just making it signed --
> this should cause no problem for the use case I was concerned about as
> it fell within the bounds I had defined, so this is basically a no-op
> patch for my usecase and I don't expect this particular failure to pop
> back up here.
> 
> Code-wise, I agree the checks I added in commit 868c9ddc182b ("swiotlb:
> add overflow checks to swiotlb_bounce") are too strict - I failed to
> consider the device minimum alignment part of swiotlb_align_offset, and
> thinking this through this can get weird.
> ... And now I'm looking again even with a big minimum alignment it's
> also too strict, so, right, let's work through an example.
> 
> 
> From my understanding of how orig_addr is computed, in the multi-block
> case we have something like this:
> 
> (+ represent device's minimum alignment, bigger blocks with | are
> IO_TLB_SIZE blocks)
> 
>          10        20        30        40        50        60
> 01234567890123456789012345678901234567890123456789012345678901234...
> |---+---+---+-block1+---+---+---|---+---+---+-block2+---+---+---|...
>        ^                               ^
>        mem->slots[n].orig_addr         mem->slots[n+1].orig_addr
>        (=7)                            (=32+7=39)
> 
> (memo of the code with your patch:
>   index = (tlb_addr - mem->start) >> IO_TLB_SHIFT;
>   orig_addr = mem->slots[index].orig_addr;
>   swiotlb_align_offset(dev, orig_addr) = orig_addr & dev min align mask & (IO_TLB_SIZE-1)
>   tlb_offset = (tlb_addr & (IO_TLB_SIZE - 1)) - swiotlb_align_offset(dev, orig_addr);
>   orig_addr += tlb_offset;
>   alloc_size -= tlb_offset;
>   vaddr = mem->vaddr + tlb_addr - mem->start
> )
> 
> So for this example we have IO_TLB_SIZE=32, dev min alignment=4,
> orig_addr's align value would be 7%4=3.
> Given say tlb_addr at 33, we'd find slot n, and compute
> tlb_offset = 1 - 3 = -2
> 
> ... And then I just don't follow anymore?
> 
> Computing the rest mechanically, for the example's sake let's say n=0
> so mem->start=7, index=0, and also set size to 4 bytes, vaddr to 0x1000.

I think your logic goes awry here.  mem->vaddr is just the virtual
address equivalent of mem->start.  See swiotlb_init_io_tlb_pool().
The computation here of vaddr is a back-handed way of getting
the virtual address equivalent of tlb_addr.  For the purposes of
this discussion, we can just use tlb_addr and ignore the virt vs.
phys difference.

Your example is saying that the originally mapped area started at
orig_addr 7.  You didn't specify the original size, but let's assume
it is at least 40.  Since you've specified that swiotlb_bounce() is
called with a tlb_addr of 33, let's assume the tlb_addr for the
full mapped area as returned by swiotlb_tbl_map_single() is 3.
It must end in 0x3 because with dev min alignment = 4, the
low order two bits of the tlb_addr of the full mapped area
must match the low order two bits of the orig_addr.

Continuing your example, subsequently swiotlb_bounce() is
called with a tlb_addr of 33, and size 4.  So we want to copy
4 bytes starting at the 30th byte (33 - 3) of the originally mapped
area, which is address 7 + 30 = 37.

In this case,
* tlb_offset = 1 - 3 = -2, as you describe above
* orig_addr = 39 + -2 = 37.  The computation uses 39 from
slot[1], not the 7 from slot[0].  This computed 37 is the
correct orig_addr to use for the memcpy().
* size is still 4.  There's no computation in swiotlb_bounce()
that changes "size".
* alloc_size is pulled from slot[1], and is adjusted by tlb_offset.
This adjusted alloc_size isn't used for anything except as a sanity
check against "size".

So I think your example works correctly with the updated
code.  Note that if a driver calls swiotlb_bounce() with a
tlb_addr that is out-of-range, swiotlb_bounce() can't detect
that.   A bogus "size" parameter _is_ detected because of
the check against the adjusted alloc_size.

Michael

> 
> vaddr = 0x1000 + 33 - 7 = 0x1000 + 26
> orig_addr = 7 + -2 = 5
> size = 4 - -2 = 6
> 
> then we'd proceed to memcpy either (vaddr, p2v(orig_addr), size) or the
> other way around, but this cannot be right:
>  - size is bigger than what was requested, I fail to see how that can be
> allowed. I'd understand a smaller size assuming swiotlb_bounce gets
> called for each interval, but not a bigger size.
>  - orig_addr is nowhere near 33.
> 
> 
> I thought this didn't make sense because of the minimum device
> alignment, but even with device alignment >= io tlb's with the very same
> example we would get
> tld_offset = 1 - 7 = -6
> now that one could make sense if we had used the following slot e.g.
> orig_addr being slot[1].orig_addr and we'd get back to 31, but that's
> not the case, and the size calculation is still off.
> 
> 
> So, long story short it took me half a day to get back into this code
> and the only thing I understand about it is that I don't understand it.
> 
> I'm sure it works most of the case because everything is nicely aligned
> (since nobody complained about my checks before, and if there's no
> warning with these the code works), but I'd require some convincing to
> give a reviewed-by tag to this patch.
> 
> Thanks for working on this though, I'll be happy to be pointed out at
> flaws in my logic or to look at another attempt...!!
> --
> Dominique

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