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

["Robin Jarry" <rjarry@redhat.com>]

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@redhat.com>