Re: Avoid speculative indirect calls in kernel

[Andrea Arcangeli]

Hi Alan,

On Thu, Jan 04, 2018 at 05:04:42PM +0000, Alan Cox wrote:
> > If you run lots of syscalls ibrs 1 ibpb 1 is much faster. If you do
> > infrequent syscalls computing a lot in kernel like I/O with large
> > buffers getting copied, ibrs 0 ibpb 2 is much faster than ibrs 1 ibpb
> > 1 (on those microcodes where ibrs 1 reduces performance a lot, not all
> > microcodes implementing SPEC_CTRL are inefficient like that).
> 
> Have you looked at whether you can measure activity and switch
> automatically between the two (or by task). It seems silly to leave
> something the machine can accurately assess toa human ?

We didn't but it'd be definitely reasonable to investigate and it's a
good idea for those CPUs where the updated microcode has to shutdown
way more than just indirect branch prediction speculation to achieve
the ibrs 1 semantics.

If the workload changes from frequent syscalls to reasonably large
read/writes and less frequent syscalls or lots of interrupts in idle
CPUs, it would work well to switch between ibrs 1 ibpb 1 and ibpb 2
ibrs 0 automatically. As long as the pattern keeps repeating for a
while... that is the question ;).

Thanks!
Andrea