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

[Eduardo Habkost]

On Mon, Jan 29, 2018 at 09:02:39PM +0000, David Woodhouse wrote:
> 
> 
> On Mon, 2018-01-29 at 12:44 -0800, Arjan van de Ven wrote:
> > On 1/29/2018 12:42 PM, Eduardo Habkost wrote:
> > > 
> > > The question is how the hypervisor could tell that to the guest.
> > > If Intel doesn't give us a CPUID bit that can be used to tell
> > > that retpolines are enough, maybe we should use a hypervisor
> > > CPUID bit for that?
> >
> > the objective is to have retpoline be safe everywhere and never use IBRS
> > (Linus was also pretty clear about that) so I'm confused by your question
> 
> The question is about all the additional RSB-frobbing and call depth
> counting and other bits that don't really even exist for Skylake yet in
> a coherent form.
> 
> If a guest doesn't have those, because it's running some future kernel
> where they *are* implemented but not enabled because at *boot* time it
> discovered it wasn't on Skylake, the question is what happens if that
> guest is subsequently migrated to a Skylake-class machine.
> 
> To which the answer is obviously "oops, sucks to be you". So yes,
> *maybe* we want a way to advertise "you might be migrated to Skylake"
> if you're booted on a pre-SKL box in a migration pool where such is
> possible. 
> 
> That question is a reasonable one, and the answer possibly the same,
> regardless of whether the plan for Skylake is to use IBRS, or all the
> hypothetical other extra stuff.

Maybe a generic "family/model/stepping/microcode really matches
the CPU you are running on" bit would be useful.  The bit could
be enabled only on host-passthrough (aka "-cpu host") mode.

If we really want to be able to migrate to host with different
CPU models (except Skylake), we could add a more specific "we
promise the host CPU is never going to be Skylake" bit.

Now, if the hypervisor is not providing any of those bits, I
would advise against trusting family/model/stepping/microcode
under a hypervisor.  Using a pre-defined CPU model (that doesn't
necessarily match the host) is very common when using KVM VM
management stacks.

-- 
Eduardo