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From: Thomas Gleixner <tglx@linutronix.de>
To: ira.weiny@intel.com, Ingo Molnar <mingo@redhat.com>,
	Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Dave Hansen <dave.hansen@linux.intel.com>
Cc: Ira Weiny <ira.weiny@intel.com>,
	Fenghua Yu <fenghua.yu@intel.com>,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-doc@vger.kernel.org, linux-nvdimm@lists.01.org,
	linux-mm@kvack.org, linux-kselftest@vger.kernel.org,
	Dan Williams <dan.j.williams@intel.com>,
	Greg KH <gregkh@linuxfoundation.org>
Subject: Re: [PATCH V3 04/10] x86/pks: Preserve the PKRS MSR on context switch
Date: Thu, 17 Dec 2020 23:43:16 +0100	[thread overview]
Message-ID: <87mtycqcjf.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <871rfoscz4.fsf@nanos.tec.linutronix.de>

On Thu, Dec 17 2020 at 15:50, Thomas Gleixner wrote:
> On Fri, Nov 06 2020 at 15:29, ira weiny wrote:
>
>> +void write_pkrs(u32 new_pkrs)
>> +{
>> +	u32 *pkrs;
>> +
>> +	if (!static_cpu_has(X86_FEATURE_PKS))
>> +		return;
>> +
>> +	pkrs = get_cpu_ptr(&pkrs_cache);
>
> So this is called from various places including schedule and also from
> the low level entry/exit code. Why do we need to have an extra
> preempt_disable/enable() there via get/put_cpu_ptr()?
>
> Just because performance in those code paths does not matter?
>
>> +	if (*pkrs != new_pkrs) {
>> +		*pkrs = new_pkrs;
>> +		wrmsrl(MSR_IA32_PKRS, new_pkrs);
>> +	}
>> +	put_cpu_ptr(pkrs);

Which made me look at the other branch of your git repo just because I
wanted to know about the 'other' storage requirements and I found this
gem:

> update_global_pkrs()
> ...
>	/*
>	 * If we are preventing access from the old value.  Force the
>	 * update on all running threads.
>	 */
>	if (((old_val == 0) && protection) ||
>	    ((old_val & PKR_WD_BIT) && (protection & PKEY_DISABLE_ACCESS))) {
>		int cpu;
>
>		for_each_online_cpu(cpu) {
>			u32 *ptr = per_cpu_ptr(&pkrs_cache, cpu);
>
>			*ptr = update_pkey_val(*ptr, pkey, protection);
>			wrmsrl_on_cpu(cpu, MSR_IA32_PKRS, *ptr);
>			put_cpu_ptr(ptr);

1) per_cpu_ptr() -> put_cpu_ptr() is broken as per_cpu_ptr() is not
   disabling preemption while put_cpu_ptr() enables it which wreckages
   the preemption count. 

   How was that ever tested at all with any debug option enabled?

   Answer: Not at all

2) How is that sequence:

	ptr = per_cpu_ptr(&pkrs_cache, cpu);
	*ptr = update_pkey_val(*ptr, pkey, protection);
	wrmsrl_on_cpu(cpu, MSR_IA32_PKRS, *ptr);

   supposed to be correct vs. a concurrent modification of the
   pkrs_cache of the remote CPU?

   Answer: Not at all

Also doing a wrmsrl_on_cpu() on _each_ online CPU is insane at best.

  A smp function call on a remote CPU takes ~3-5us when the remote CPU
  is not idle and can immediately respond. If the remote CPU is deep in
  idle it can take up to 100us depending on C-State it is in.

  Even if the remote CPU is not not idle and just has interrupts
  disabled for a few dozen of microseconds this adds up.

  So on a 256 CPU system depending on the state of the remote CPUs this
  stalls the CPU doing the update for anything between 1 and 25ms worst
  case.

  Of course that also violates _all_ CPU isolation mechanisms.

  What for?

  Just for the theoretical chance that _all_ remote CPUs have
  seen that global permission and have it still active?

  You're not serious about that, right?

The only use case for this in your tree is: kmap() and the possible
usage of that mapping outside of the thread context which sets it up.

The only hint for doing this at all is:

    Some users, such as kmap(), sometimes requires PKS to be global.

'sometime requires' is really _not_ a technical explanation.

Where is the explanation why kmap() usage 'sometimes' requires this
global trainwreck in the first place and where is the analysis why this
can't be solved differently?

Detailed use case analysis please.

Thanks,

        tglx




  reply	other threads:[~2020-12-17 22:44 UTC|newest]

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-06 23:28 [PATCH V3 00/10] PKS: Add Protection Keys Supervisor (PKS) support V3 ira.weiny
2020-11-06 23:28 ` [PATCH V3 01/10] x86/pkeys: Create pkeys_common.h ira.weiny
2020-11-06 23:29 ` [PATCH V3 02/10] x86/fpu: Refactor arch_set_user_pkey_access() for PKS support ira.weiny
2020-11-06 23:29 ` [PATCH V3 03/10] x86/pks: Add PKS defines and Kconfig options ira.weiny
2020-11-06 23:29 ` [PATCH V3 04/10] x86/pks: Preserve the PKRS MSR on context switch ira.weiny
2020-12-17 14:50   ` Thomas Gleixner
2020-12-17 22:43     ` Thomas Gleixner [this message]
2020-12-18 13:57       ` Thomas Gleixner
2020-12-18 19:20         ` Dan Williams
2020-12-18 21:06           ` Thomas Gleixner
2020-12-18 21:58             ` Dan Williams
2020-12-18 22:44               ` Thomas Gleixner
2020-12-18 19:42         ` Ira Weiny
2020-12-18 20:10           ` Dave Hansen
2020-12-18 21:30           ` Thomas Gleixner
2020-12-18  4:05     ` Ira Weiny
2020-12-17 20:41   ` [NEEDS-REVIEW] " Dave Hansen
2020-12-18  4:10     ` Ira Weiny
2020-12-18 15:33       ` Dave Hansen
2020-11-06 23:29 ` [PATCH V3 05/10] x86/entry: Pass irqentry_state_t by reference ira.weiny
2020-11-15 18:58   ` Thomas Gleixner
2020-11-16 18:49     ` Ira Weiny
2020-11-16 20:36       ` Thomas Gleixner
2020-11-24  6:09   ` [PATCH V3.1] entry: " ira.weiny
2020-12-11 22:14     ` Andy Lutomirski
2020-12-16  1:32       ` Ira Weiny
2020-12-16  2:09         ` Andy Lutomirski
2020-12-17  0:38           ` Ira Weiny
2020-12-17 13:07       ` Thomas Gleixner
2020-12-17 13:19         ` Peter Zijlstra
2020-12-17 15:35           ` Andy Lutomirski
2020-12-17 16:58     ` Thomas Gleixner
2020-11-06 23:29 ` [PATCH V3 06/10] x86/entry: Preserve PKRS MSR across exceptions ira.weiny
2020-12-17 15:28   ` Thomas Gleixner
2020-11-06 23:29 ` [PATCH V3 07/10] x86/fault: Report the PKRS state on fault ira.weiny
2020-11-06 23:29 ` [PATCH V3 08/10] x86/pks: Add PKS kernel API ira.weiny
2020-12-23 20:39   ` Randy Dunlap
2020-11-06 23:29 ` [PATCH V3 09/10] x86/pks: Enable Protection Keys Supervisor (PKS) ira.weiny
2020-11-06 23:29 ` [PATCH V3 10/10] x86/pks: Add PKS test code ira.weiny
2020-12-17 20:55   ` Dave Hansen
2020-12-18  4:05     ` Ira Weiny
2020-12-18 16:59       ` Dan Williams
2020-12-07 22:14 ` [PATCH V3 00/10] PKS: Add Protection Keys Supervisor (PKS) support V3 Ira Weiny
2020-12-08 15:55   ` Thomas Gleixner
2020-12-08 17:22     ` Ira Weiny

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