linux-kernel - Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict Indirect Branch Speculation

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Date:   Sat, 27 Jan 2018 07:55:10 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Konrad Rzeszutek Wilk <konrad@...nok.org>
Cc:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Liran Alon <liran.alon@...cle.com>,
        Laura Abbott <labbott@...hat.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Janakarajan Natarajan <Janakarajan.Natarajan@....com>,
        Borislav Petkov <bp@...e.de>,
        "Mallick, Asit K" <asit.k.mallick@...el.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        KarimAllah Ahmed <karahmed@...zon.de>,
        Peter Anvin <hpa@...or.com>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        Ingo Molnar <mingo@...hat.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        Joerg Roedel <joro@...tes.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        KVM list <kvm@...r.kernel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict
 Indirect Branch Speculation

On 01/27/2018 05:42 AM, Konrad Rzeszutek Wilk wrote:
> On Fri, Jan 26, 2018 at 07:11:47PM +0000, Hansen, Dave wrote:
>> The need for RSB stuffing in all the various scenarios and what the heck it actually mitigates is freakishly complicated.  I've tried to write it all down in one place: https://goo.gl/pXbvBE
> Thank you for sharing that.
> 
> One question on the third from the top (' RSB Stuff (16) After
> irq/nmi/#PF/...').
> 
> It says that :"Return from interrupt path (more than 16 deep) can empty
> RSB".
> 
> Just to clarify - you mean all the returns ('ret') that are happening after
> we call do_IRQ and the stack unwinds - but before we do an 'iret' correct?

Correct.  The RSB is not used or updated by iret.