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From: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
To: peterz@infradead.org
Cc: hpa@zytor.com, Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@suse.de>,
	Andy Lutomirski <luto@kernel.org>,
	x86@kernel.org, Dave Hansen <dave.hansen@intel.com>,
	Tony Luck <tony.luck@intel.com>,
	Cathy Zhang <cathy.zhang@intel.com>,
	Fenghua Yu <fenghua.yu@intel.com>,
	Kyung Min Park <kyung.min.park@intel.com>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	Sean Christopherson <sean.j.christopherson@intel.com>,
	linux-kernel@vger.kernel.org,
	Ricardo Neri <ricardo.neri@intel.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	linux-edac@vger.kernel.org, frederic@kernel.org
Subject: Re: [PATCH 4/4] x86/cpu: Use SERIALIZE in sync_core() when available
Date: Mon, 27 Jul 2020 21:41:14 -0700
Message-ID: <20200728044114.GA24129@ranerica-svr.sc.intel.com> (raw)
In-Reply-To: <20200727133020.GN119549@hirez.programming.kicks-ass.net>

On Mon, Jul 27, 2020 at 03:30:20PM +0200, peterz@infradead.org wrote:
> On Mon, Jul 27, 2020 at 03:05:36PM +0200, peterz@infradead.org wrote:
> > Yeah, I'm not sure.. the 'funny' thing is that typically call
> > sync_core() from an IPI anyway. And the synchronous broadcast IPI is by
> > far the most expensive part of that.
> > 
> > Something like this...
> > 
> > diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> > index 20e07feb4064..528e049ee1d9 100644
> > --- a/arch/x86/kernel/alternative.c
> > +++ b/arch/x86/kernel/alternative.c
> > @@ -989,12 +989,13 @@ void *text_poke_kgdb(void *addr, const void *opcode, size_t len)
> >  
> >  static void do_sync_core(void *info)
> >  {
> > -	sync_core();
> > +	/* IRET implies sync_core() */
> >  }
> >  
> >  void text_poke_sync(void)
> >  {
> >  	on_each_cpu(do_sync_core, NULL, 1);
> > +	sync_core();
> >  }
> >  
> >  struct text_poke_loc {
> 
> So 'people' have wanted to optimize this for NOHZ_FULL and I suppose
> virt as well.
> 
> IFF VMENTER is serializing, I suppose we can simply do something like:
> 
> bool text_poke_cond(int cpu, void *info)
> {
> 	/*
> 	 * If we observe the vCPU is preempted, it will do VMENTER
> 	 * no point in sending an IPI to SERIALIZE.
> 	 */
> 	return !vcpu_is_preempted(cpu);
> }
> 
> void text_poke_sync(void)
> {
> 	smp_call_function_many_cond(cpu_possible_mask,
> 				    do_sync_core, NULL, 1, text_poke_cond);
> 	sync_core();
> }
> 
> The 'same' for NOHZ_FULL, except we need to cmpxchg a value such that
> if the cmpxchg() succeeds we know the CPU is in userspace and will
> SERIALIZE on the next entry. Much like kvm_flush_tlb_others().
> 
> 
> Anyway, that's all hand-wavey.. I'll let someone that cares about those
> things write actual patches :-)

I think I got a little lost here. If I understand correctly, there are
two alternatives to implement support for serialize better:

  a) alternative(IRET_TO_SELF, SERIALIZE, X86_FEATURE_SERIALIZE); or
  b) asm volatile("1:.byte 0xf, 0x1, 0xe8;2:" _ASM_EXTABLE(1b:2b)

a) would be the traditional and simpler solution. b) would rely on
causing an #UD and getting an IRET on existing hardware b) would need some
more optimization work when handling the exception and a few reworks on
the poke patching code.

Which option should I focus on? Which option would be more desirable/better?

Thanks and BR,
Ricardo

  reply index

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20200727043132.15082-1-ricardo.neri-calderon@linux.intel.com>
2020-07-27  4:31 ` [PATCH 1/4] x86/cpufeatures: Add enumeration for SERIALIZE instruction Ricardo Neri
2020-07-27  4:31 ` [PATCH 2/4] x86/cpu: Relocate sync_core() to sync_core.h Ricardo Neri
2020-07-27  4:31 ` [PATCH 3/4] x86/cpu: Refactor sync_core() for readability Ricardo Neri
2020-07-27  4:31 ` [PATCH 4/4] x86/cpu: Use SERIALIZE in sync_core() when available Ricardo Neri
2020-07-27  5:55   ` hpa
2020-07-27  6:00     ` hpa
2020-07-27  8:36     ` peterz
2020-07-27 12:49       ` hpa
2020-07-27 13:05         ` peterz
2020-07-27 13:30           ` peterz
2020-07-28  4:41             ` Ricardo Neri [this message]
2020-07-28  8:50               ` peterz
2020-07-27 16:27     ` Luck, Tony
2020-07-27  8:20   ` peterz
2020-07-27 12:47     ` hpa
2020-07-28  0:55       ` Ricardo Neri
2020-07-28  0:36     ` Ricardo Neri
2020-07-27  8:23   ` peterz

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