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

[Linus Torvalds]

On Mon, Jan 29, 2018 at 5:32 PM, Arjan van de Ven <arjan@xxxxxxxxxxxxxxx> wrote:
>
> the most simple solution is that we set the internal feature bit in Linux
> to turn on the "stuff the RSB" workaround is we're on a SKL *or* as a guest
> in a VM.

That sounds reasonable.

However, wouldn't it be even better to extend on the current cpuid
model, and actually have some real architectural bits in there.

Maybe it could be a bit in that IA32_ARCH_CAPABILITIES MSR. Say, add a
bit #2 that says "ret falls back on BTB".

Then that bit basically becomes the "Skylake bit". Hmm?

                  Linus