From mboxrd@z Thu Jan  1 00:00:00 1970
From: Arjan van de Ven <arjan@linux.intel.com>
Subject: Re: [RFC,05/10] x86/speculation: Add basic IBRS support
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Date: Tue, 30 Jan 2018 05:54:15 -0800
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Cc: David Woodhouse <dwmw2@infradead.org>,
        Eduardo Habkost <ehabkost@redhat.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>,
        Borislav Petkov <bp@suse.de>,
        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>,
        Janakarajan Natarajan <Janakarajan.Natarajan@amd.com>,
        Joerg Roedel <joro@8bytes.org>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Laura Abbott <labbott@redhat.com>,
        Masami Hir
To: Linus Torvalds <torvalds@linux-foundation.org>
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On 1/29/2018 7:32 PM, Linus Torvalds wrote:
> On Mon, Jan 29, 2018 at 5:32 PM, Arjan van de Ven <arjan@linux.intel.com> wrote:
>>
>> the most simple solution is that we set the internal feature bit in Linux
>> to turn on the "stuff the RSB" workaround is we're on a SKL *or* as a guest
>> in a VM.
> 
> That sounds reasonable.
> 
> However, wouldn't it be even better to extend on the current cpuid
> model, and actually have some real architectural bits in there.
> 
> Maybe it could be a bit in that IA32_ARCH_CAPABILITIES MSR. Say, add a
> bit #2 that says "ret falls back on BTB".
> 
> Then that bit basically becomes the "Skylake bit". Hmm?

we can try to do that, but existing systems don't have that, and then we
get in another long thread here about weird lists of stuff ;-)