Re: [PATCH 1/3] drm/panfrost: Simplify lock_region calculation

[Steven Price]

On 23 August 2021 22:09:08 BST, Alyssa Rosenzweig <alyssa@xxxxxxxxxxxxx> wrote:
>> > In lock_region, simplify the calculation of the region_width parameter.
>> > This field is the size, but encoded as log2(ceil(size)) - 1.
>> > log2(ceil(size)) may be computed directly as fls(size - 1). However, we
>> > want to use the 64-bit versions as the amount to lock can exceed
>> > 32-bits.
>> > 
>> > This avoids undefined behaviour when locking all memory (size ~0),
>> > caught by UBSAN.
>> 
>> It might have been useful to mention what it is that UBSAN specifically
>> picked up (it took me a while to spot) - but anyway I think there's a
>> bigger issue with it being completely wrong when size == ~0 (see below).
>
>Indeed. I've updated the commit message in v2 to explain what goes
>wrong (your analysis was spot on, but a mailing list message is more
>ephermal than a commit message). I'll send out v2 tomorrow assuming
>nobody objects to v1 in the mean time.
>
>Thanks for the review.
>
>> There is potentially a third bug which kbase only recently attempted to
>> fix. The lock address is effectively rounded down by the hardware (the
>> bottom bits are ignored). So if you have mask=(1<<region_width)-1 but
>> (iova & mask) != ((iova + size) & mask) then you are potentially failing
>> to lock the end of the intended region. kbase has added some code to
>> handle this:
>> 
>> > 	/* Round up if some memory pages spill into the next region. */
>> > 	region_frame_number_start = pfn >> (lockaddr_size_log2 - PAGE_SHIFT);
>> > 	region_frame_number_end =
>> > 	    (pfn + num_pages - 1) >> (lockaddr_size_log2 - PAGE_SHIFT);
>> > 
>> > 	if (region_frame_number_start < region_frame_number_end)
>> > 		lockaddr_size_log2 += 1;
>> 
>> I guess we should too?
>
>Oh, I missed this one. Guess we have 4 bugs with this code instead of
>just 3, yikes. How could such a short function be so deeply and horribly
>broken? ����
>
>Should I add a fourth patch to the series to fix this?

Yes please!

Thanks,
Steve