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Date:   Fri, 3 May 2019 09:40:40 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        David Laight <David.Laight@...lab.com>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        Theodore Ts'o <tytso@....edu>,
        Eric Biggers <ebiggers3@...il.com>,
        "ebiggers@...gle.com" <ebiggers@...gle.com>,
        "herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
        Peter Zijlstra <peterz@...radead.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "jannh@...gle.com" <jannh@...gle.com>,
        "Perla, Enrico" <enrico.perla@...el.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall


> On May 2, 2019, at 9:43 AM, Ingo Molnar <mingo@...nel.org> wrote:
> 
> 
> * Andy Lutomirski <luto@...nel.org> wrote:
> 
>>> 8 gigabits/sec sounds good throughput in principle, if there's no
>>> scalability pathologies with that.
>> 
>> The latency is horrible.
> 
> Latency would be amortized via batching anyway, so 8 gigabits/sec 
> suggests something on the order of magnitude of 4 bits per cycle, right? 
> With 64 bits extraction at a time that would be 16 cycles per 64-bit 
> word, which isn't too bad, is it?

I haven’t really dug in, but some Googling suggests that the 8Gbps figure is what you get with all cores doing RDRAND.  It sounds like the actual RDRAND instruction doesn’t pipeline.

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

True.

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