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
