[lkml]   [2018]   [Jan]   [31]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [RFC,05/10] x86/speculation: Add basic IBRS support infrastructure
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 <> 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.


 \ /
  Last update: 2018-01-31 16:02    [W:0.093 / U:1.424 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site