Quoting Chang, Bruce (2020-08-15 03:16:58)
> On 8/14/2020 5:36 PM, Chang, Bruce wrote:
> >
> >>>> @@ -2498,9 +2498,22 @@ invalidate_csb_entries(const u64 *first,
> >>>> const u64 *last)
> >>>> */
> >>>> static inline bool gen12_csb_parse(const u64 *csb)
> >>>> {
> >>>> - u64 entry = READ_ONCE(*csb);
> >>>> - bool ctx_away_valid = GEN12_CSB_CTX_VALID(upper_32_bits(entry));
> >>>> - bool new_queue =
> >>>> + bool ctx_away_valid;
> >>>> + bool new_queue;
> >>>> + u64 entry;
> >>>> +
> >>>> + /* XXX HSD */
> >>>> + entry = READ_ONCE(*csb);
> >>>> + if (unlikely(entry == -1)) {
> >>>> + preempt_disable();
> >>>> + if (wait_for_atomic_us((entry = READ_ONCE(*csb)) !=
> >>>> -1, 50))
> >>>> + GEM_WARN_ON("50us CSB timeout");
> >>> Out tests showed that 10us is not long enough, but 20us worked well. So
> >>> 50us should be good enough.
> >
> > Just realized this may not fully work, as one of the common issue we
> > run into is that higher 32bit is updated from the HW, but lower 32bit
> > update at a later time: meaning the csb will read like
> > 0xFFFFFFFF:xxxxxxxx (low:high) . So this check (!= -1) can still pass
> > but with a partial invalid csb status. So, we may need to check each
> > 32bit separately.
> >
> After tested, with the new 64bit read, the above issue never happened so
> far. So, it seems this only applicable to 32bit read (CSB updated
> between the two lower and high 32bit reads). Assuming the HW 64bit CSB
> update is also atomic, the above code should be fine.
Fortunately for all the platforms we care about here, READ_ONCE(u64)
will be a single 64b read and so both lower/upper dwords will be pulled
from the same bus transfer. We really need a compiler warning for when
READ_ONCE() is not a singular atomic operation. atomic64_t has too much
connotation with cross-core atomicity for my liking when dealing with
[cacheable] mmio semantics.
-Chris
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