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Date:   Mon, 29 Jan 2018 23:30:14 -0200
From:   Eduardo Habkost <ehabkost@redhat.com>
To:     David Dunn <ddunn@vmware.com>
Cc:     Jim Mattson <jmattson@google.com>, Andi Kleen <ak@linux.intel.com>,
        Arjan van de Ven <arjan@linux.intel.com>,
        KarimAllah Ahmed <karahmed@amazon.de>,
        "Wilson, Matt" <msw@amazon.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        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>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        "Jorgensen, Bryan" <bryan.jorgensen@intel.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>,
        "Dr. David Alan Gilbert" <dgilbert@redhat.com>,
        Fred Jacobs <fjacobs@vmware.com>,
        David Woodhouse <dwmw2@infradead.org>
Subject: Re: [RFC,05/10] x86/speculation: Add basic IBRS support
 infrastructure
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On Tue, Jan 30, 2018 at 01:20:52AM +0000, David Dunn wrote:
> Eduardo,
> 
> This is why it would be good to have a CPUID bit that says:
> "apply SkyLake RSB stuffing."  That's preferable to "trust FMS"
> for VMware.

Agreed it would be more useful than "trust FMS".  However, I
believe a "no need to apply Skylake RSB stuffing" bit (which I
called "we promise we won't migrate to Skylake" previously) would
allow guests to enable safer behavior by default under older
hypervisors that don't support this bit.

> 
> If Intel defines such a feature flag, sets it on SkyLake, and
> Linux uses it... that would be very helpful for VMware.
> 
> I won't speak for GCE and AWS.  But hopefully they can indicate
> whether it would help them as well.

I agree that having a standard flag on the CPUID space to specify
that would be very helpful.

> 
> If Intel cannot define/implement such a flag on SkyLake, then
> maybe the engineers on this email could define a flag in the
> hypervisor specific CPUID space.  Linux would need to query
> that flag if it sees CPUID[1].ECX[31] set.  That's not as nice
> since it makes detection on bare metal and virtualization
> platforms different, but it better than keying off FMS.

Agreed.

-- 
Eduardo