On 1/25/23 07:29, Dave Hansen wrote: > There's another part here which I think was recently added to the > documentation: > > Intel expects the performance impact of this mode may be > significantly higher on future processors. > > That's _meant_ to be really scary and keep folks from turning this on by > default, aka. what this patch does. Your new CPU will be really slow if > you turn this on! Boo! *If* we go forward with this patch's approach in the kernel, I think we should at least consider what the kernel will do in a future where there are two classes of systems: 1. Ice Lake era ones where DOITM=1 is cheap enough that it can on by default. 2. "Future processors" where DOITM=1 by default is too costly. Maybe for #2 we set DOITM=0 in the kernel. Maybe we add per-task controls. But, there *is* DOITM cost where the large fleets are going to be tempted to turn it off somehow, somewhere. The kernel will be better off if we can design that in now.
