linux-kernel - Re: Avoid speculative indirect calls in kernel

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Message-ID: <20180104170442.710aa4c4@alans-desktop>
Date:   Thu, 4 Jan 2018 17:04:42 +0000
From:   Alan Cox <gnomes@...rguk.ukuu.org.uk>
To:     Andrea Arcangeli <aarcange@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
        "pavel@....cz" <pavel@....cz>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "andi@...stfloor.org" <andi@...stfloor.org>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>,
        "gregkh@...ux-foundation.org" <gregkh@...ux-foundation.org>
Subject: Re: Avoid speculative indirect calls in kernel

> If you run lots of syscalls ibrs 1 ibpb 1 is much faster. If you do
> infrequent syscalls computing a lot in kernel like I/O with large
> buffers getting copied, ibrs 0 ibpb 2 is much faster than ibrs 1 ibpb
> 1 (on those microcodes where ibrs 1 reduces performance a lot, not all
> microcodes implementing SPEC_CTRL are inefficient like that).

Have you looked at whether you can measure activity and switch
automatically between the two (or by task). It seems silly to leave
something the machine can accurately assess toa human ?

Alan