Re: [PATCH 0/5] x86/speculation: Disable IBRS when idle

[Peter Zijlstra]

On Fri, Jun 16, 2023 at 10:14:52PM +0200, Robin Jarry wrote:
> Waiman Long, Jun 16, 2023 at 21:59:
> > For Intel processors that need to turn on IBRS to protect against
> > Spectre v2 and Retbleed, the IBRS bit in the SPEC_CTRL MSR affects
> > the performance of the whole core even if only one thread is turning
> > it on when running in the kernel. For user space heavy applications,
> > the performance impact of occasionally turning IBRS on during syscalls
> > shouldn't be significant. Unfortunately, that is not the case when the
> > sibling thread is idling in the kernel. In that case, the performance
> > impact can be significant.
> >
> > When DPDK is running on an isolated CPU thread processing network packets
> > in user space while its sibling thread is idle. The performance of the
> > busy DPDK thread with IBRS on and off in the sibling idle thread are:
> >
> >                                 IBRS on               IBRS off
> >                                 -------               --------
> >   packets/second:                  7.8M                  10.4M
> >   avg tsc cycles/packet:         282.26                 209.86
> >
> > This is a 25% performance degradation. The test system is a Intel Xeon
> > 4114 CPU @ 2.20GHz.
> >
> > This patch series turns off IBRS when in various idle mode to eliminate
> > the performance impact of the idling thread on its busy sibling thread.
> 
> Hi Longman,
> 
> thanks a lot for the quick turnaround on this issue.
> 
> Tested-by: Robin Jarry <rjarry@xxxxxxxxxx>

I can't see the patches -- they didn't arrive in my mailbox nor can I
find them in the archive, in fact this here mail is the only evidence
they exist at all.

However, did you all see intel_idle_ibrs() and how that is selected for
C6 and up?

What exactly isn't working there?

Also, instead of investing more in this IBRS trainwreck, did you all try
call-depth-stuffing ?