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

[Thomas Gleixner]

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.

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".

Good luck with making all that work.

Thanks,

	tglx