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Date:   Wed, 31 Jan 2018 13:00:47 -0200
From:   Eduardo Habkost <ehabkost@redhat.com>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     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>,
        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 Hiramatsu <mhiramat@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        KVM list <kvm@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Subject: Re: [RFC,05/10] x86/speculation: Add basic IBRS support
 infrastructure
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On Wed, Jan 31, 2018 at 11:15:50AM +0100, Thomas Gleixner wrote:
> On Wed, 31 Jan 2018, Christophe de Dinechin wrote:
> > > On 30 Jan 2018, at 21:46, Alan Cox <gnomes@lxorguk.ukuu.org.uk> wrote:
> > > 
> > >> If you are ever going to migrate to Skylake, I think you should just
> > >> always tell the guests that you're running on Skylake. That way the
> > >> guests will always assume the worst case situation wrt Specte.
> > > 
> > > Unfortunately if you do that then guest may also decide to use other
> > > Skylake hardware features and pop its clogs when it finds out its actually
> > > running on Westmere or SandyBridge.
> > > 
> > > So you need to be able to both lie to the OS and user space via cpuid and
> > > also have a second 'but do skylake protections' that only mitigation
> > > aware software knows about.
> > 
> > Yes. The most desirable lie is different depending on whether you want to
> > allow virtualization features such as migration (where you’d gravitate
> > towards a CPU with less features) or whether you want to allow mitigation
> > (where you’d rather present the most fragile CPUID, probably Skylake).
> > 
> > Looking at some recent patches, I’m concerned that the code being added
> > often assumes that the CPUID is the correct way to get that info.
> > I do not think this is correct. You really want specific information about
> > the host CPUID, not whatever KVM CPUID emulation makes up.
> 
> That wont cut it. If you have a heterogenous farm of systems, then you need:
> 
>   - All CPUs have to support IBRS/IBPB or at least hte hypervisor has to
>     pretend they do by providing fake MRS for that
> 
>   - Have a 'force IBRS/IBPB' mechanism so the guests don't discard it due
>     to missing CPU feature bits.

If all your hosts have IBRS/IBPB, you enable it.  If some of your
hosts don't have IBRS/IBPB, you don't expose it to the guest (and
deal with the consequences of not applying updates to your
hardware).  Where's the problem?

> 
> Though this gets worse. You have to make sure that the guest keeps _ALL_
> sorts of mitigation mechanisms enabled and does not decide to disable
> retpolines because IBRS/IBPB are "available".

If IBRS/IBPB are reported as available to the guest, the VM
management system will ensure the VM won't be migrated to a host
that doesn't have it.  That's a pretty basic feature of VM
management stacks.

Exactly the same could happen to a "(non-)skylake bit".  The host
reports a feature (or a bug fix) as available to a guest, and
then the system ensures you won't migrate to a host that doesn't
provide that feature.

The problem I see here is that Linux guests currently have no way
to tell if it needs to enable Skylake-specific mitigations or
not.  Unless you make Linux always enable skylake mitigations if
seeing the hypervisor bit, you will need the hypervisor to
provide more useful information than f/m/s.

-- 
Eduardo