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SubjectRe: Improve retpoline for Skylake
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> On Jan 15, 2018, at 12:26 AM, Jon Masters <jcm@jonmasters.org> 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?
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