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.
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