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From: Adrian Hunter <adrian.hunter@intel.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Andi Kleen <ak@linux.intel.com>, Nadav Amit <namit@vmware.com>,
	Ingo Molnar <mingo@redhat.com>, Andy Lutomirski <luto@kernel.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Edward Cree <ecree@solarflare.com>,
	"H . Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>,
	Nadav Amit <nadav.amit@gmail.com>, X86 ML <x86@kernel.org>,
	Paolo Abeni <pabeni@redhat.com>, Borislav Petkov <bp@alien8.de>,
	David Woodhouse <dwmw@amazon.co.uk>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	songliubraving@fb.com
Subject: Re: [RFC v2 0/6] x86: dynamic indirect branch promotion
Date: Tue, 8 Jan 2019 12:01:11 +0200
Message-ID: <306d38fb-7ce6-a3ec-a351-6c117559ebaa@intel.com> (raw)
In-Reply-To: <20190108092559.GA6808@hirez.programming.kicks-ass.net>

On 8/01/19 11:25 AM, Peter Zijlstra wrote:
> On Tue, Jan 08, 2019 at 09:47:18AM +0200, Adrian Hunter wrote:
>> On 7/01/19 6:32 PM, Peter Zijlstra wrote:
>>> On Thu, Jan 03, 2019 at 02:18:15PM -0800, Andi Kleen wrote:
>>>> Nadav Amit <namit@vmware.com> writes:
>>>>>
>>>>> - Do we use periodic learning or not? Josh suggested to reconfigure the
>>>>>   branches whenever a new target is found. However, I do not know at
>>>>>   this time how to do learning efficiently, without making learning much
>>>>>   more expensive.
>>>>
>>>> FWIW frequent patching will likely completely break perf Processor Trace
>>>> decoding, which needs a somewhat stable kernel text image to decode the
>>>> traces generated by the CPU. Right now it relies on kcore dumped after
>>>> the trace usually being stable because jumplabel changes happen only
>>>> infrequently. But if you start patching frequently this assumption will
>>>> break.
>>>>
>>>> You would either need a way to turn this off, or provide
>>>> updates for every change to the trace, so that the decoder can
>>>> keep track.
>>>
>>> I'm thining it would be entirely possible to create and feed text_poke
>>> events into the regular (!aux) buffer which can be timestamp correlated
>>> to the PT data.
>>
>> To rebuild kernel text from such events would require a starting point.
>> What is the starting point?  The problem with kcore is that people can
>> deconfig it without realising it is needed to enable the tracing of kernel
>> self-modifying code.  It would be nice if it was all tied together, so that
>> if someone selects the ability to trace kernel self-modifying code, then all
>> the bits needed are also selected.  Perhaps we should expose another ELF
>> image that contains only kernel executable code, and take the opportunity to
>> put the symbols in it also.
> 
> Meh; you always need a magic combo of CONFIG symbols to make stuff work.
> We don't even have a CONFIG symbol for PT, so if you really care you
> should probably start there.
> 
> If you want symbols; what stops us from exposing kallsyms in kcore as
> is?
> 
>> Also what about BPF jitted code?  Will it always fit in an event?  I was
>> thinking of trying to add a way to prevent temporarily the unload of modules
>> or jitted code, which would be a good-enough solution for now.
> 
> We're working on BPF and kallsym events, those should, esp. when
> combined with kcore, allow you to extract the actual instructions.

The problem is that the jitted code gets freed from memory, which is why I
suggested the ability to pin it for a while.

  reply index

Thread overview: 42+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-12-31  7:21 Nadav Amit
2018-12-31  7:21 ` [RFC v2 1/6] x86: introduce kernel restartable sequence Nadav Amit
2018-12-31 20:08   ` Andy Lutomirski
2018-12-31 21:12     ` Nadav Amit
2019-01-03 22:21   ` Andi Kleen
2019-01-03 22:29     ` Nadav Amit
2019-01-03 22:48       ` Andi Kleen
2019-01-03 22:52         ` Nadav Amit
2019-01-03 23:40           ` Andi Kleen
2019-01-03 23:56             ` Nadav Amit
2019-01-04  0:34   ` hpa
2018-12-31  7:21 ` [RFC v2 2/6] objtool: ignore instructions Nadav Amit
2018-12-31  7:21 ` [RFC v2 3/6] x86: patch indirect branch promotion Nadav Amit
2018-12-31  7:21 ` [RFC v2 4/6] x86: interface for accessing indirect branch locations Nadav Amit
2018-12-31  7:21 ` [RFC v2 5/6] x86: learning and patching indirect branch targets Nadav Amit
2018-12-31 20:05   ` Andy Lutomirski
2018-12-31 21:07     ` Nadav Amit
2018-12-31  7:21 ` [RFC v2 6/6] x86: outline optpoline Nadav Amit
2018-12-31 19:51 ` [RFC v2 0/6] x86: dynamic indirect branch promotion Andy Lutomirski
2018-12-31 19:53   ` Nadav Amit
2019-01-03 18:10     ` Josh Poimboeuf
2019-01-03 18:30       ` Nadav Amit
2019-01-03 20:31         ` Josh Poimboeuf
2019-01-03 22:18 ` Andi Kleen
2019-01-07 16:32   ` Peter Zijlstra
2019-01-08  7:47     ` Adrian Hunter
2019-01-08  9:25       ` Peter Zijlstra
2019-01-08 10:01         ` Adrian Hunter [this message]
2019-01-08 10:10           ` Peter Zijlstra
2019-01-08 17:27             ` Andi Kleen
2019-01-08 18:28               ` Nadav Amit
2019-01-08 19:01                 ` Peter Zijlstra
2019-01-08 20:47                   ` Nadav Amit
2019-01-08 20:53                     ` Andi Kleen
2019-01-09 10:35                     ` Peter Zijlstra
2019-08-29  8:23                       ` Tracing text poke / kernel self-modifying code (Was: Re: [RFC v2 0/6] x86: dynamic indirect branch promotion) Adrian Hunter
2019-08-29  8:53                         ` Peter Zijlstra
2019-08-29  9:40                           ` Adrian Hunter
2019-08-29 11:46                             ` Peter Zijlstra
2019-09-12  7:00                               ` Adrian Hunter
2019-09-12 12:17                                 ` hpa
2019-01-08 18:57               ` [RFC v2 0/6] x86: dynamic indirect branch promotion Peter Zijlstra

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