Re: [RFC] #MC mess

From: Andy Lutomirski
Date: Wed Feb 19 2020 - 10:20:36 EST

> On Feb 19, 2020, at 7:05 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> ïOn Wed, Feb 19, 2020 at 09:21:15AM -0500, Steven Rostedt wrote:
>>> On Wed, 19 Feb 2020 09:15:41 +0100
>>> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>>>>> Tony, etc, can you ask your Intel contacts who care about this kind of
>>>>> thing to stop twiddling their thumbs and FIX IT? The easy fix is
>>>>> utterly trivial. Add a new instruction IRET_NON_NMI. It does
>>>>> *exactly* the same thing as IRET except that it does not unmask NMIs.
>>>>> (It also doesn't unmask NMIs if it faults.) No fancy design work.
>>>>> Future improvements can still happen on top of this.
>>> Yes please! Of course, we're stuck with the existing NMI entry crap
>>> forever because legacy, but it would make all things NMI so much saner.
>> What would be nice is to have a NMI_IRET, that is defined as something
>> that wont break legacy CPUs. Where it could be just a nop iret, or maybe
>> if possible a "lock iret"? That is, not have a IRET_NON_NMI, as that
>> would be all over the place, but just the iret for NMI itself. As
>> that's in one place.
> I don't think that matters much; alternatives should be able to deal
> with all that either which way around.

Agreed. That being said, kernels without alternatives could prefer the variant where a CR4 bit makes regular IRET leave NMIs masked and a new IRET instruction (or LOCK IRET, I suppose) unmasks them.