linux-kernel - Re: [RFC,05/10] x86/speculation: Add basic IBRS support infrastructure

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Message-ID: <20180131144441.GK26425@localhost.localdomain>
Date:   Wed, 31 Jan 2018 12:44:41 -0200
From:   Eduardo Habkost <ehabkost@...hat.com>
To:     "Dr. David Alan Gilbert" <dgilbert@...hat.com>
Cc:     Borislav Petkov <bp@...e.de>, Thomas Gleixner <tglx@...utronix.de>,
        Christophe de Dinechin <christophe.de.dinechin@...il.com>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        KarimAllah Ahmed <karahmed@...zon.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ashok Raj <ashok.raj@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "H . Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Janakarajan Natarajan <Janakarajan.Natarajan@....com>,
        Joerg Roedel <joro@...tes.org>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Laura Abbott <labbott@...hat.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        KVM list <kvm@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [RFC,05/10] x86/speculation: Add basic IBRS support
 infrastructure

On Wed, Jan 31, 2018 at 02:04:49PM +0000, Dr. David Alan Gilbert wrote:
> * Borislav Petkov (bp@...e.de) wrote:
> > On Wed, Jan 31, 2018 at 12:30:36PM +0000, Dr. David Alan Gilbert wrote:
> > > Indeed, it's only for this weird case where you suddenly need to change
> > > it.
> > 
> > No, there's more:
> > 
> > 	.name = "Broadwell-noTSX",
> > 	.name = "Haswell-noTSX",
> 
> Haswell came out and we made the CPU definition, and then got a
> microcode update that removed the feature.
> 
> So the common feature of noTSX and IBRS is that they're the only two
> cases where a CPU has released and then the flags have changed later.

Also, if anybody don't like it, users can already specify, e.g.,
"Broadwell,-hle,-rtm" or "Skylake,+spec_ctrl".

QEMU only adds have the -noTSX and -IBRS CPU for convenience of
management systems that don't know how to check/configure
individual CPU features.  We're working with libvirt and
OpenStack folks to make this kind of trick unnecessary.

-- 
Eduardo