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Subject: Re: [RFC v2 1/6] x86: introduce kernel restartable sequence
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <87va35e61a.fsf@linux.intel.com>
Date:   Thu, 3 Jan 2019 14:29:36 -0800
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>
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Message-Id: <263A441C-F062-491F-9E95-F00FA2092A99@gmail.com>
References: <20181231072112.21051-1-namit@vmware.com>
 <20181231072112.21051-2-namit@vmware.com> <87va35e61a.fsf@linux.intel.com>
To:     Andi Kleen <ak@linux.intel.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Jan 3, 2019, at 2:21 PM, Andi Kleen <ak@linux.intel.com> wrote:
>=20
> Nadav Amit <namit@vmware.com> writes:
>=20
> I see another poor man's attempt to reinvent TSX.
>=20
>> It is sometimes beneficial to have a restartable sequence - very few
>> instructions which if they are preempted jump to a predefined point.
>>=20
>> 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.
>=20
> You cannot just assume something like that. x86 is a constantly
> evolving architecture. The prefix might well have meaning at
> some point.
>=20
> Before doing something like that you would need to ask the CPU
> vendors to reserve the sequence you're using for software use.

Ok=E2=80=A6 I=E2=80=99ll 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=E2=80=99t 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 =
=E2=80=9Cinjected=E2=80=9D 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 =
=E2=80=9Cunpatchable=E2=80=9D,
increase the code size and slow performance.

Anyhow, I=E2=80=99ll give more thought. Ideas are welcomed.