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

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Date:   Mon, 29 Jan 2018 23:08:32 -0200
From:   Eduardo Habkost <ehabkost@...hat.com>
To:     David Dunn <ddunn@...are.com>
Cc:     Arjan van de Ven <arjan@...ux.intel.com>,
        KarimAllah Ahmed <karahmed@...zon.de>,
        "Wilson, Matt" <msw@...zon.com>,
        "linux-kernel@...r.kernel.org" <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>,
        Borislav Petkov <bp@...e.de>,
        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>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Fred Jacobs <fjacobs@...are.com>,
        Jim Mattson <jmattson@...gle.com>,
        David Woodhouse <dwmw2@...radead.org>
Subject: Re: [RFC,05/10] x86/speculation: Add basic IBRS support
 infrastructure

On Mon, Jan 29, 2018 at 10:29:28PM +0000, David Dunn wrote:
> On Mon, 2018-01-29 at 13:45:07 -0800, Eduardo Habkost wrote:
> 
> > Maybe a generic "family/model/stepping/microcode really matches
> > the CPU you are running on" bit would be useful.  The bit could
> > be enabled only on host-passthrough (aka "-cpu host") mode.
> > 
> > If we really want to be able to migrate to host with different
> > CPU models (except Skylake), we could add a more specific "we
> > promise the host CPU is never going to be Skylake" bit.
> > 
> > Now, if the hypervisor is not providing any of those bits, I
> > would advise against trusting family/model/stepping/microcode
> > under a hypervisor.  Using a pre-defined CPU model (that doesn't
> > necessarily match the host) is very common when using KVM VM
> > management stacks.
> > 
> 
> Eduardo,
> 
> I don't see how this is possible in a modern virtualization
> environment.
>  
> Under VMware, a VM will be migrated to SkyLake if one is in the
> cluster and supports the features exposed to the VM.  This can
> occur for suspend/resume as well.
> 
> The migration pool isn't a constant.  Hosts can be added to a
> cluster and VMs can be instructed to move across clusters.  So
> there doesn't need to be a SkyLake around when the VM powers on
> in order for it to eventually end up on a SkyLake.

If this is the case for your deployment, this means the guest
must never assume it won't run on a Skylake host (even if f/m/s
is not Skylake), doesn't it?  Then the hypervisor won't set the
"we promise the host CPU is never going to be Skylake" bit.

> 
> Even if we expose bit to indicate that FMS matches the
> underlying host, when does the guest know to query that?  The
> VM can be moved at any point in time, including after the guest
> asks if FMS matches host.

If the VM can be moved at any point of time to a different model
of host CPU, this means you won't tell the guest it can trust
f/m/s because it doesn't represent the underlying host.  You
won't set the "f/m/s/m really matches the host CPU" bit.

On both scenarios you describe above, it sounds like Linux must
assume it could migrated to a Skylake host at any moment.  This
is exactly why I'm proposing those extra bits.

-- 
Eduardo