linux-kernel - RE: [PATCH 0/7] IBRS patch series

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Date:   Fri, 5 Jan 2018 14:45:48 +0000
From:   "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Woodhouse <dwmw2@...radead.org>
CC:     Tim Chen <tim.c.chen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Andrea Arcangeli" <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 0/7] IBRS patch series





> -----Original Message-----
> From: Paolo Bonzini [mailto:paolo.bonzini@...il.com] On Behalf Of Paolo
> Bonzini
> Sent: Friday, January 05, 2018 3:32 AM
> To: Van De Ven, Arjan <arjan.van.de.ven@...el.com>; Linus Torvalds
> <torvalds@...ux-foundation.org>; David Woodhouse <dwmw2@...radead.org>
> Cc: Tim Chen <tim.c.chen@...ux.intel.com>; Thomas Gleixner
> <tglx@...utronix.de>; Andy Lutomirski <luto@...nel.org>; Greg KH
> <gregkh@...uxfoundation.org>; Hansen, Dave <dave.hansen@...el.com>;
> Andrea Arcangeli <aarcange@...hat.com>; Andi Kleen <ak@...ux.intel.com>;
> Linux Kernel Mail

> > personally I am comfortable with retpoline on Skylake, but I would
> > like to have IBRS as an opt in for the paranoid.
> >
> 
> IBRS actually seems to perform more than decently on Skylake with the
> microcode updates we're shipping in RHEL---much more so than Broadwell
> or Haswell.  Can you confirm that this is expected?

Retpoline outperforms IBRS even on skylake family (including the client kabylake etc)
by some real margin, but IBRS is certainly not the end of the world either... more a case of 'we can do even faster'. The difference also gets bigger in retpoline's favor for older generations cpus.