Mark Cave-Ayland <mark.cave-ayl...@ilande.co.uk> writes: > On 15/03/17 11:14, Alex Bennée wrote: > >> BALATON Zoltan <bala...@eik.bme.hu> writes: >> >>> On Tue, 14 Mar 2017, Alex Bennée wrote: >>>> So from a single-threaded -smp guest case there should be no difference >>>> in behaviour. >> <snip> >>>> However this shouldn't affect >>>> anything in the single-threaded world. >>> >>> I think we have a single CPU and thread for these ppc machines here so >>> I'm not sure how this could be relevant. >>> >>>> However delaying tlb_flushes() could certainly expose/hide stuff that is >>>> accessing the dirty mechanism. tlb_flush itself now takes the tb_lock() to >>>> avoid racing with the TB invalidation logic. The act of the flush will >>>> certainly wipe all existing SoftMMU entries and force a re-load on each >>>> memory access. >>>> >>>> So is the dirty status of memory being read from outside a vCPU >>>> execution context? >>> >>> Like from the display controller models that use >>> memory_region_get_dirty() to check if the frambuffer needs to be >>> updated? But all display adaptors seem to do this and the problem was >>> only seem on ppc so it may be related to something ppc specific. >> >> So this accesses the memory_region API which is under RCU control. >> AFAIUI this should mean the dirty status may be read late (e.g. next >> update) but should never be incorrect (e.g. miss a dirtying operation). > > AFAICT check_tlb_flush() gets passed a global parameter which if set > true invalidates the TLB across all CPU TLBs rather than just the local > CPU TLB - but then in this case we're only running with a single CPU so > I can't see how this is relevant.
Not quite. tlb_flush used to take a global flag but it ignored it. It has been removed in recent updates to the cputlb API ;-) > Have you been able to reproduce the artifacts locally at all? I'm > wondering if once the icount fixup patches are in, it might be easier to > debug if enabling icount causes the artifacts to appear in a > deterministic manner. I have, and I can make them go away as well by forcing a full update. See my other longer email for a description of what I think is happening. Now I just need a neat and upstreamable fix ;-) -- Alex Bennée
