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

[Dr. David Alan Gilbert]

* Thomas Gleixner (tglx@xxxxxxxxxxxxx) wrote:
> On Wed, 31 Jan 2018, Christophe de Dinechin wrote:
> > > On 30 Jan 2018, at 21:46, Alan Cox <gnomes@xxxxxxxxxxxxxxxxxxx> 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 youâd gravitate
> > towards a CPU with less features) or whether you want to allow mitigation
> > (where youâd rather present the most fragile CPUID, probably Skylake).
> > 
> > Looking at some recent patches, Iâm 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.

That half is the easy bit, we've already got that (thanks to Eduardo),
QEMU has -IBRS variants of CPU types, so if you start a VM with
-cpu Broadwell-IBRS  it'll get advertised to the guest as having IBRS;
and (with appropriate flags) the management layers will only allow that
to be started on hosts that support IBRS and wont allow migration
between hosts with and without it.

> 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".

This is what's different with this set; it's all coming down to sets
of heuristics which include CPU model etc, rather than just a 'we've got
a feature, use it'.

Dave

> Good luck with making all that work.
> 
> Thanks,
> 
> 	tglx

--
Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK