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. In general, making these kinds of decisions based on F/M/S is probably unwise when running in a VM. On Mon, Jan 29, 2018 at 1:02 PM, David Woodhouse <dwmw2@xxxxxxxxxxxxx> 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.
