[Intel-gfx] [PATCH] drm/i915: Disable atomics in L3 for gen9

[Francisco Jerez]

Chris Wilson <chris at chris-wilson.co.uk> writes:

> Quoting Francisco Jerez (2019-07-23 23:19:13)
>> Chris Wilson <chris at chris-wilson.co.uk> writes:
>> 
>> > Quoting Tvrtko Ursulin (2019-07-22 12:41:36)
>> >> 
>> >> On 20/07/2019 15:31, Chris Wilson wrote:
>> >> > Enabling atomic operations in L3 leads to unrecoverable GPU hangs, as
>> >> > the machine stops responding milliseconds after receipt of the reset
>> >> > request [GDRT]. By disabling the cached atomics, the hang do not occur
>> >> > and we presume the GPU would reset normally for similar hangs.
>> >> > 
>> >> > Reported-by: Jason Ekstrand <jason at jlekstrand.net>
>> >> > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110998
>> >> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
>> >> > Cc: Jason Ekstrand <jason at jlekstrand.net>
>> >> > Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
>> >> > Cc: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
>> >> > ---
>> >> > Jason reports that Windows is not clearing L3SQCREG4:22 and does not
>> >> > suffer the same GPU hang so it is likely some other w/a that interacts
>> >> > badly. Fwiw, these 3 are the only registers I could find that mention
>> >> > atomic ops (and appear to be part of the same chain for memory access).
>> >> 
>> >> Bit-toggling itself looks fine to me and matches what I could find in 
>> >> the docs. (All three bits across three registers should be equal.)
>> >> 
>> >> What I am curious about is what are the other consequences of disabling 
>> >> L3 atomics? Performance drop somewhere?
>> >
>> > The test I have where it goes from dead to passing, that's a considerable
>> > performance improvement ;)
>> >
>> > I imagine not being able to use L3 for atomics is pretty dire, whether that
>> > has any impact, I have no clue.
>> >
>> > It is still very likely that we see this because we are doing something
>> > wrong elsewhere.
>> 
>> This reminds me of f3fc4884ebe6ae649d3723be14b219230d3b7fd2 followed by
>> d351f6d94893f3ba98b1b20c5ef44c35fc1da124 due to the massive impact (of
>> the order of 20x IIRC) using the L3 turned out to have on the
>> performance of HDC atomics, on at least that platform.  It seems
>> unfortunate that we're going to lose L3 atomics on Gen9 now, even though
>> it's only buffer atomics which are broken IIUC, and even though the
>> Windows driver is somehow getting away without disabling them.  Some of
>> our setup must be wrong either in the kernel or in userspace...  Are
>> these registers at least whitelisted so userspace can re-enable L3
>> atomics once the problem is addressed?  Wouldn't it be a more specific
>> workaround for userspace to simply use a non-L3-cacheable MOCS for
>> (rarely used) buffer surfaces, so it could benefit from L3 atomics
>> elsewhere?
>
> If it was the case that disabling L3 atomics was the only way to prevent
> the machine lockup under this scenario, then I think it is
> unquestionably the right thing to do, and we could not leave it to
> userspace to dtrt. We should never add non-context saved unsafe
> registers to the whitelist (if setting a register may cause data
> corruption or worse in another context/process, that is bad) despite our
> repeated transgressions. However, there's no evidence to say that it does
> prevent the machine lockup as it prevents the GPU hang that lead to the
> lockup on reset.
>
> Other than GPGPU requiring a flush around every sneeze, I did not see
> anything in the gen9 w/a list that seemed like a match. Nevertheless, I
> expect there is a more precise w/a than a blanket disable.
> -Chris

Supposedly there is a more precise one (setting the surface state MOCS
to UC for buffer images), but it relies on userspace doing the right
thing for the machine not to lock up.  There is a good chance that the
reason why L3 atomics hang on such buffers is ultimately under userspace
control, in which case we'll eventually have to undo the programming
done in this patch in order to re-enable L3 atomics once the problem is
addressed.  That means that userspace will have the freedom to hang the
machine hard once again, which sounds really bad, but it's no real news
for us (*cough* HSW *cough*), and it might be the only way to match the
performance of the Windows driver.

What can we do here?  Add an i915 option to enable performance features
that can lead to the system hanging hard under malicious (or
incompetent) userspace programming?  Probably only the user can tell
whether the trade-off between performance and security of the system is
acceptable...
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