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

[Eduardo Habkost]

On Mon, Jan 29, 2018 at 01:37:05PM -0800, Jim Mattson wrote:
> For GCE, "you might be migrated to Skylake" is pretty much a
> certainty. Even if you're in a zone that doesn't currently have
> Skylake machines, chances are pretty good that it will have Skylake
> machines some day in the not-too-distant future.

This kind of scenario is why I suggest a "we promise you're not
going to be migrated to Skylake" bit instead a "you may be
migrated to Skylake" bit.  The hypervisor could prevent migration
to Skylake hosts if management software chose to enable this bit,
and guests would choose the safest option (i.e. assume the worst)
if running on older hypervisors that don't set the bit.

> 
> In general, making these kinds of decisions based on F/M/S is probably
> unwise when running in a VM.

Certainly.  That's why I suggest not trusting f/m/s unless the
hypervisor is explicitly saying it's accurate.

-- 
Eduardo