Re: Improve retpoline for Skylake

[Andy Lutomirski]

> On Jan 15, 2018, at 12:26 AM, Jon Masters <jcm@xxxxxxxxxxxxxx> wrote:
> 
>> On 01/12/2018 05:03 PM, Henrique de Moraes Holschuh wrote:
>> On Fri, 12 Jan 2018, Andi Kleen wrote:
>>>> Skylake still loses if it takes an SMI, right? 
>>> 
>>> SMMs are usually rare, especially on servers, and are usually
>>> not very predictible, and even if you have
>> 
>> FWIW, a data point: SMIs can be generated on demand by userspace on
>> thinkpad laptops, but they will be triggered from within a kernel
>> context.  I very much doubt this is a rare pattern...
> 
> Sure. Just touch some "legacy" hardware that the vendor emulates in a
> nasty SMI handler. It's definitely not acceptable to assume that SMIs
> can't be generated under the control of some malicious user code.
> 
> Our numbers on Skylake weren't bad, and there seem to be all kinds of
> corner cases, so again, it seems as if IBRS is the safest choice.
> 

And keep in mind that SMIs generally hit all CPUs at once, making them extra nasty.

Can we get firmware vendors to refill the return buffer just before RSM?