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From: Peter Zijlstra <peterz@infradead.org>
To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: gor@linux.ibm.com, jpoimboe@redhat.com, jikos@kernel.org,
	mbenes@suse.cz, pmladek@suse.com, mingo@kernel.org,
	linux-kernel@vger.kernel.org, joe.lawrence@redhat.com,
	fweisbec@gmail.com, tglx@linutronix.de, hca@linux.ibm.com,
	svens@linux.ibm.com, sumanthk@linux.ibm.com,
	live-patching@vger.kernel.org, paulmck@kernel.org,
	rostedt@goodmis.org, x86@kernel.org
Subject: Re: [RFC][PATCH v2 11/11] context_tracking,x86: Fix text_poke_sync() vs NOHZ_FULL
Date: Thu, 21 Oct 2021 22:18:59 +0200	[thread overview]
Message-ID: <20211021201859.GX174703@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <20211021195709.GA22422@fuller.cnet>

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

> > > > +             /* 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.
> 
> > 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.

> > 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.

  reply	other threads:[~2021-10-21 20:21 UTC|newest]

Thread overview: 51+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-29 15:17 [PATCH v2 00/11] sched,rcu,context_tracking,livepatch: Improve livepatch task transitions for idle and NOHZ_FULL Peter Zijlstra
2021-09-29 15:17 ` [PATCH v2 01/11] sched: Improve try_invoke_on_locked_down_task() Peter Zijlstra
2021-09-29 15:17 ` [PATCH v2 02/11] sched,rcu: Rework try_invoke_on_locked_down_task() Peter Zijlstra
2021-09-29 15:17 ` [PATCH v2 03/11] sched,livepatch: Use task_call_func() Peter Zijlstra
2021-10-05 11:40   ` Petr Mladek
2021-10-05 14:03     ` Peter Zijlstra
2021-10-06  8:59   ` Miroslav Benes
2021-09-29 15:17 ` [PATCH v2 04/11] sched: Simplify wake_up_*idle*() Peter Zijlstra
2021-10-13 14:32   ` Qian Cai
2021-10-19  3:47     ` Qian Cai
2021-10-19  8:56       ` Peter Zijlstra
2021-10-19  9:10         ` Peter Zijlstra
2021-10-19 15:32           ` Qian Cai
2021-10-19 15:50             ` Peter Zijlstra
2021-10-19 19:22               ` Qian Cai
2021-10-19 20:27                 ` Peter Zijlstra
     [not found]   ` <CGME20211022134630eucas1p2e79e2816587d182c580459d567c1f2a9@eucas1p2.samsung.com>
2021-10-22 13:46     ` Marek Szyprowski
2021-09-29 15:17 ` [PATCH v2 05/11] sched,livepatch: Use wake_up_if_idle() Peter Zijlstra
2021-10-05 12:00   ` Petr Mladek
2021-10-06  9:16   ` Miroslav Benes
2021-10-07  9:18     ` Vasily Gorbik
2021-10-07 10:02       ` Peter Zijlstra
2021-10-13 19:37   ` Arnd Bergmann
2021-10-14 10:42     ` Peter Zijlstra
2021-09-29 15:17 ` [RFC][PATCH v2 06/11] context_tracking: Prefix user_{enter,exit}*() Peter Zijlstra
2021-09-29 15:17 ` [RFC][PATCH v2 07/11] context_tracking: Add an atomic sequence/state count Peter Zijlstra
2021-09-29 15:17 ` [RFC][PATCH v2 08/11] context_tracking,rcu: Replace RCU dynticks counter with context_tracking Peter Zijlstra
2021-09-29 18:37   ` Paul E. McKenney
2021-09-29 19:09     ` Peter Zijlstra
2021-09-29 19:11     ` Peter Zijlstra
2021-09-29 19:13     ` Peter Zijlstra
2021-09-29 19:24       ` Peter Zijlstra
2021-09-29 19:45         ` Paul E. McKenney
2021-09-29 18:54   ` Peter Zijlstra
2021-09-29 15:17 ` [RFC][PATCH v2 09/11] context_tracking,livepatch: Dont disturb NOHZ_FULL Peter Zijlstra
2021-10-06  8:12   ` Petr Mladek
2021-10-06  9:04     ` Peter Zijlstra
2021-10-06 10:29       ` Petr Mladek
2021-10-06 11:41         ` Peter Zijlstra
2021-10-06 11:48         ` Miroslav Benes
2021-09-29 15:17 ` [RFC][PATCH v2 10/11] livepatch: Remove klp_synchronize_transition() Peter Zijlstra
2021-10-06 12:30   ` Petr Mladek
2021-09-29 15:17 ` [RFC][PATCH v2 11/11] context_tracking,x86: Fix text_poke_sync() vs NOHZ_FULL Peter Zijlstra
2021-10-21 18:39   ` Marcelo Tosatti
2021-10-21 18:40     ` Marcelo Tosatti
2021-10-21 19:25     ` Peter Zijlstra
2021-10-21 19:57       ` Marcelo Tosatti
2021-10-21 20:18         ` Peter Zijlstra [this message]
2021-10-26 18:19           ` Marcelo Tosatti
2021-10-26 19:38             ` Peter Zijlstra
2021-09-29 18:03 ` [PATCH v2 00/11] sched,rcu,context_tracking,livepatch: Improve livepatch task transitions for idle and NOHZ_FULL Paul E. McKenney

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