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Date:   Fri, 3 May 2019 16:17:28 +0000
From:   "Reshetova, Elena" <elena.reshetova@...el.com>
To:     Ingo Molnar <mingo@...nel.org>,
        David Laight <David.Laight@...LAB.COM>
CC:     Andy Lutomirski <luto@...nel.org>, 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

> * David Laight <David.Laight@...LAB.COM> wrote:
> 
> > It has already been measured - it is far too slow.
> 
> I don't think proper buffering was tested, was it? Only a per syscall
> RDRAND overhead which I can imagine being not too good.
> 

Well, I have some numbers, but I am struggling to understand one
aspect there. 

So, this is how it looks when PAGE_TABLE_ISOLATION is off:

base:     Simple syscall: 0.0516 microseconds
rdrand (calling every 8 syscalls): Simple syscall: 0.0795 microseconds
get_random_bytes (4096 bytes buffer):  Simple syscall: 0.0597 microseconds

But then it looks like this with PAGE_TABLE_ISOLATION is on:

base:                                                               Simple syscall: 0.1761 microseconds
get_random_bytes (4096 bytes buffer): Simple syscall: 0.1793 microseconds
get_random_bytes (64 bytes buffer):     Simple syscall: 0.1866 microseconds
rdrand (calling every 8 syscalls):   Simple syscall: 0.3131 microseconds

So, suddenly calling rdrand is much more pricey... 
Either smth is really weird going on when PAGE_TABLE is enabled, or
I managed to do smth wrongly (no idea what although). I will continue 
Investigating. 

Best Regards,
Elena.

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