Re: [RFC][PATCH v2 11/11] context_tracking,x86: Fix text_poke_sync() vs NOHZ_FULL

On Thu, Oct 21, 2021 at 04:57:09PM -0300, Marcelo Tosatti wrote:

quoted

Pretty much everything in noinstr is magical, we just have to think
harder there (and possibly start writing more comments there).

mds_user_clear_cpu_buffers happens after sync_core, in your patchset, 
if i am not mistaken.

Of course it does, mds_user_clear_cpu_buffers() is on exit, the
sync_core() is on entry.

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+             /* NMI happens here and must still do/finish CT_WORK_n */
+             sync_core();

But after the discussion with you, it seems doing the TLB checking 
and (also sync_core) checking very late/very early on exit/entry 
makes things easier to review.

I don't know about late, it must happen *very* early in entry. The
sync_core() must happen before any self-modifying code gets called
(static_branch, static_call, etc..) with possible exception of the
context_tracking static_branch.

The TLBi must also happen super early, possibly while still on the
entry stack (since the task stack is vmap'ed).

But will it be ever be freed/remapped from other CPUs while the task
is running?

Probably not, still something we need to be really careful with.

quoted

We currently don't run C
code on the entry stack, that needs quite a bit of careful work to make
happen.

Was thinking of coding in ASM after (as early as possible) the write to 
switch to kernel CR3:

No, we're not going to add new feature to ASM. You'll just have to wait
until all that gets lifted to C.

 Kernel entry:
 -------------

       cpu = smp_processor_id();

       if (isolation_enabled(cpu)) {
               reqs = atomic_xchg(&percpudata->user_kernel_state, IN_KERNEL_MODE);
               if (reqs & CPU_REQ_FLUSH_TLB)
			flush_tlb_all();
               if (reqs & CPU_REQ_SYNC_CORE)
			sync_core();
       }                           

Exit to userspace (as close to write to CR3 with user pagetable
pointer):
 -----------------

       cpu = smp_processor_id();

       if (isolation_enabled(cpu)) {
               atomic_or(IN_USER_MODE, &percpudata->user_kernel_state);
       }

You think that is a bad idea (in ASM, not C) ? 

Those atomics are a bad idea and not goig to happen.

quoted

We're not going to add an atomic to context tracking. There is one, we
just got to extract/share it with RCU.

Again, to avoid kernel TLB flushes you'd have to ensure:

I know how it works, but we're not going to add a second atomic to
entry/exit. RCU has one in there, that's going to be it. Again, we just
got to extract/share.