Subject: Re: [PATCH v6 00/10] Retpoline: Avoid speculative indirect calls in
 kernel
To: Paul Turner <pjt@google.com>, David Woodhouse <dwmw@amazon.co.uk>
CC: Andi Kleen <ak@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Kees Cook <keescook@google.com>,
        Rik van Riel <riel@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Jiri Kosina <jikos@kernel.org>,
        One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
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From: Andrew Cooper <andrew.cooper3@citrix.com>
Message-ID: <37ca6fc5-78d5-5750-051f-a712343d4a8f@citrix.com>
Date: Mon, 8 Jan 2018 11:16:24 +0000
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On 08/01/18 10:42, Paul Turner wrote:
> A sequence for efficiently refilling the RSB is:
>     mov $8, %rax;
>     .align 16;
>    3: call 4f;
>   3p: pause; call 3p;
>      .align 16;
>   4: call 5f;
>   4p: pause; call 4p;
>      .align 16;
>    5: dec %rax;
>       jnz 3b;
>       add $(16*8), %rsp;
> This implementation uses 8 loops, with 2 calls per iteration.  This is
> marginally faster than a single call per iteration.  We did not
> observe useful benefit (particularly relative to text size) from
> further unrolling.  This may also be usefully split into smaller (e.g.
> 4 or 8 call)  segments where we can usefully pipeline/intermix with
> other operations.  It includes retpoline type traps so that if an
> entry is consumed, it cannot lead to controlled speculation.  On my
> test system it took ~43 cycles on average.  Note that non-zero
> displacement calls should be used as these may be optimized to not
> interact with the RSB due to their use in fetching RIP for 32-bit
> relocations.

Guidance from both Intel and AMD still states that 32 calls are required
in general.  Is your above code optimised for a specific processor which
you know the RSB to be smaller on?

~Andrew