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From: Nadav Amit <nadav.amit@gmail.com>
To: Andi Kleen <ak@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>, Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.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>, X86 ML <x86@kernel.org>,
	Paolo Abeni <pabeni@redhat.com>, Borislav Petkov <bp@alien8.de>,
	David Woodhouse <dwmw@amazon.co.uk>
Subject: Re: [RFC v2 1/6] x86: introduce kernel restartable sequence
Date: Thu, 3 Jan 2019 14:29:36 -0800
Message-ID: <263A441C-F062-491F-9E95-F00FA2092A99@gmail.com> (raw)
In-Reply-To: <87va35e61a.fsf@linux.intel.com>

> On Jan 3, 2019, at 2:21 PM, Andi Kleen <ak@linux.intel.com> wrote:
> 
> Nadav Amit <namit@vmware.com> writes:
> 
> I see another poor man's attempt to reinvent TSX.
> 
>> It is sometimes beneficial to have a restartable sequence - very few
>> instructions which if they are preempted jump to a predefined point.
>> 
>> To provide such functionality on x86-64, we use an empty REX-prefix
>> (opcode 0x40) as an indication for instruction in such a sequence. Before
>> calling the schedule IRQ routine, if the "magic" prefix is found, we
>> call a routine to adjust the instruction pointer.  It is expected that
>> this opcode is not in common use.
> 
> You cannot just assume something like that. x86 is a constantly
> evolving architecture. The prefix might well have meaning at
> some point.
> 
> Before doing something like that you would need to ask the CPU
> vendors to reserve the sequence you're using for software use.

Ok… I’ll try to think about another solution. Just note that this is just
used as a hint to avoid unnecessary lookups. (IOW, nothing will break if the
prefix is used.)

> You're doing the equivalent of patching a private system call
> into your own kernel without working with upstream, don't do that.

I don’t understand this comment though. Can you please explain?

> Better to find some other solution to do the restart.
> How about simply using a per cpu variable? That should be cheaper
> anyways.

The problem is that the per-cpu variable needs to be updated after the call
is executed, when we are already not in the context of the “injected” code.
I can increase it before the call, and decrease it after return - but this
can create (in theory) long periods in which the code is “unpatchable”,
increase the code size and slow performance.

Anyhow, I’ll give more thought. Ideas are welcomed.


  reply index

Thread overview: 42+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-12-31  7:21 [RFC v2 0/6] x86: dynamic indirect branch promotion 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 [this message]
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
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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