Re: [PATCH V3] riscv: asid: Fixup stale TLB entry cause application crash

[Sergey Matyukevich]

Hi Guo Ren,

> > > After use_asid_allocator is enabled, the userspace application will
> > > crash by stale TLB entries. Because only using cpumask_clear_cpu without
> > > local_flush_tlb_all couldn't guarantee CPU's TLB entries were fresh.
> > > Then set_mm_asid would cause the user space application to get a stale
> > > value by stale TLB entry, but set_mm_noasid is okay.
> >
> > ... [snip]
> >
> > > +     /*
> > > +      * The mm_cpumask indicates which harts' TLBs contain the virtual
> > > +      * address mapping of the mm. Compared to noasid, using asid
> > > +      * can't guarantee that stale TLB entries are invalidated because
> > > +      * the asid mechanism wouldn't flush TLB for every switch_mm for
> > > +      * performance. So when using asid, keep all CPUs footmarks in
> > > +      * cpumask() until mm reset.
> > > +      */
> > > +     cpumask_set_cpu(cpu, mm_cpumask(next));
> > > +     if (static_branch_unlikely(&use_asid_allocator)) {
> > > +             set_mm_asid(next, cpu);
> > > +     } else {
> > > +             cpumask_clear_cpu(cpu, mm_cpumask(prev));
> > > +             set_mm_noasid(next);
> > > +     }
> > >  }
> >
> > I observe similar user-space crashes on my SMP systems with enabled ASID.
> > My attempt to fix the issue was a bit different, see the following patch:
> >
> > https://lore.kernel.org/linux-riscv/20220829205219.283543-1-geomatsi@xxxxxxxxx/
> >
> > In brief, the idea was borrowed from flush_icache_mm handling:
> > - keep track of CPUs not running the task
> > - perform per-ASID TLB flush on such CPUs only if the task is switched there
> >
> > Your patch also works fine in my tests fixing those crashes. I have a
> > question though, regarding removed cpumask_clear_cpu. How CPUs no more
> > running the task are removed from its mm_cpumask ? If they are not
> > removed, then flush_tlb_mm/flush_tlb_page will broadcast unnecessary
> > TLB flushes to those CPUs when ASID is enabled.
> A task would be migrated to any CPU by the scheduler. So keeping TLB
> contents synced with cpumask_set/clear needs additional tlb_flush just
> like noasid, and your patch still follows that style. The worth of
> ASID is avoiding tlb_flush during the context switch. Yes, my patch
> would increase some tlb_flush IPI costs. But when mapping is stable,
> no tlb_flush is needed during the switch_mm (Hackbench would be
> beneficiary because no more TLB flush is needed at his hot point
> path). Here are my points:
>  - We copied the arm64 globally unique asid mechanism into riscv,
> which depends on hardware broadcast TLB flush. My fixup patch is
> closer to the original principle design, proven in the arm64 world.
>  - If riscv continues local TLB flush hw design in ISA spec, please
> try x86's per-CPU array of ASID. But that is a significant change;
> let's fix the current issue with the smallest patch first.
> 
> In the end, thx your review and test.

By the way, how did you verify the patch ? Do you have any good
reproducer for this issue ?

Regards,
Sergey