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.
Regards,
Sergey