Hi Gentlemen,

I have installed Linux (RHEL variant) with Spectre and Meltdown fix and see ~15% performance drop when I enabled pti_enabled, ibpb_enabled and ibrs_enabled.
Then experimented one by one and found that PTI and IBPB has no impact but all 15% drop is coming from IBRS. This I see clearly when enable/disable IBRS.
I am doing it runtime “# echo 1 > /sys/kernel/debug/x86/ibrs_enabled”. Why only IBRS is dropping the performance but not other 2? i.e PTI and IBPB.

Also I saw in web some suggestion on how to reduce performance drop. We can reduce performance drop when we set “spectre_v2=retpoline,ibrs_user”and set
IBRS to 3, i.e "#echo 3 > /sys/kernel/debug/x86/ibrs_enabled".

I was able to set spectre_v2=retpoline,ibrs_user and rebooted the OS and I could see this reflected in /proc/cmdline. But RHEL is not allowing to set IBRS to 3 with
echo after rebooting. Can anyone suggest what could be the issue. I am using Kernal 3.10.0-693.17.1.el7.x86_64 with RHEL 7.4.

Want to enable Spectre and Meltdown correction. Any suggestions to drop any one of these settings (PTI, IBPB and IBRS) to minimize performance drop or any other
Recommendations/suggestions to minimize performance drop from 15%.

Regards

You can't have it both ways. To prevent pipeline exploitation you have to turn-off the pipelining and suffer the performance hit.

And, personally, I'm not sure that it's really worth it.

Since you're using RHEL, have you contacted Red Hat support??? Because not only do they have an application available to detect if your hardware has this flaw, but also patches for the kernel to mitigate it while limiting performance problems.

Give them a call, and they can direct you to those resources.

Thanks sundialsvcs.
Yes i do feel the same. I am trying to see what is the level of impact on system behavior and is it worth enabling this at the cost of heavy performance.

Regards

Yes i am in touch with RHEL and looking forward for some suggestions on benefits vs losses with with IBRS. I personally not very convinced to enable IBRS.

Regards

Finally i found it is possible to set IBRS to 3 with Kernal version 3.10.0-693.21.1.el7.x86_64. Sofar I am using Kernal version 3.10.0-693.17.1.el7.x86_64.
I will do more tests and investigation with new Kernal and post the result for the benefit of full Linux community.

And unless people pay for RHEL and apply the resources given, the steps aren't going to be of much benefit. Also, it is spelled "kernel".