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Date:   Wed, 31 Jan 2018 12:44:41 -0200
From:   Eduardo Habkost <ehabkost@redhat.com>
To:     "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc:     Borislav Petkov <bp@suse.de>, Thomas Gleixner <tglx@linutronix.de>,
        Christophe de Dinechin <christophe.de.dinechin@gmail.com>,
        Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        David Woodhouse <dwmw2@infradead.org>,
        Arjan van de Ven <arjan@linux.intel.com>,
        KarimAllah Ahmed <karahmed@amazon.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andi Kleen <ak@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        Ashok Raj <ashok.raj@intel.com>,
        Asit Mallick <asit.k.mallick@intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "H . Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
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Subject: Re: [RFC,05/10] x86/speculation: Add basic IBRS support
 infrastructure
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On Wed, Jan 31, 2018 at 02:04:49PM +0000, Dr. David Alan Gilbert wrote:
> * Borislav Petkov (bp@suse.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