Re: [PATCH] x86: enable Data Operand Independent Timing Mode

Dave Hansen
2023-01-25T16:15:21Z

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.