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Subject: Re: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20190502164325.GA115950@gmail.com>
Date:   Fri, 3 May 2019 09:40:40 -0700
Cc:     Andy Lutomirski <luto@kernel.org>,
        David Laight <David.Laight@aculab.com>,
        "Reshetova, Elena" <elena.reshetova@intel.com>,
        Theodore Ts'o <tytso@mit.edu>,
        Eric Biggers <ebiggers3@gmail.com>,
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        "herbert@gondor.apana.org.au" <herbert@gondor.apana.org.au>,
        Peter Zijlstra <peterz@infradead.org>,
        "keescook@chromium.org" <keescook@chromium.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "jpoimboe@redhat.com" <jpoimboe@redhat.com>,
        "jannh@google.com" <jannh@google.com>,
        "Perla, Enrico" <enrico.perla@intel.com>,
        "mingo@redhat.com" <mingo@redhat.com>,
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        "tglx@linutronix.de" <tglx@linutronix.de>,
        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>
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To:     Ingo Molnar <mingo@kernel.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On May 2, 2019, at 9:43 AM, Ingo Molnar <mingo@kernel.org> wrote:
>=20
>=20
> * Andy Lutomirski <luto@kernel.org> wrote:
>=20
>>> 8 gigabits/sec sounds good throughput in principle, if there's no
>>> scalability pathologies with that.
>>=20
>> The latency is horrible.
>=20
> Latency would be amortized via batching anyway, so 8 gigabits/sec=20
> suggests something on the order of magnitude of 4 bits per cycle, right?=20=

> With 64 bits extraction at a time that would be 16 cycles per 64-bit=20
> word, which isn't too bad, is it?

I haven=E2=80=99t really dug in, but some Googling suggests that the 8Gbps f=
igure is what you get with all cores doing RDRAND.  It sounds like the actua=
l RDRAND instruction doesn=E2=80=99t pipeline.

> Making it "optional" is not really a technical argument in any way=20
> though, either distros enable it in which case it's a de-facto default=20
> setting, or they don't, in which case it de-facto almost doesn't exist.
>=20
>=20

True.=