On 03/07/2020 06:04 AM, Qian Cai wrote: > > >> On Mar 6, 2020, at 7:03 PM, Anshuman Khandual <Anshuman.Khandual@xxxxxxx> wrote: >> >> Hmm, set_pte_at() function is not preferred here for these tests. The idea >> is to avoid or atleast minimize TLB/cache flushes triggered from these sort >> of 'static' tests. set_pte_at() is platform provided and could/might trigger >> these flushes or some other platform specific synchronization stuff. Just > > Why is that important for this debugging option? Primarily reason is to avoid TLB/cache flush instructions on the system during these tests that only involve transforming different page table level entries through helpers. Unless really necessary, why should it emit any TLB/cache flush instructions ? > >> wondering is there specific reason with respect to the soft lock up problem >> making it necessary to use set_pte_at() rather than a simple WRITE_ONCE() ? > > Looks at the s390 version of set_pte_at(), it has this comment, > vmaddr); > > /* > * Certain architectures need to do special things when PTEs > * within a page table are directly modified. Thus, the following > * hook is made available. > */ > > I can only guess that powerpc could be the same here. This comment is present in multiple platforms while defining set_pte_at(). Is not 'barrier()' here alone good enough ? Else what exactly set_pte_at() does as compared to WRITE_ONCE() that avoids the soft lock up, just trying to understand.
