Messages in this thread | | | Date | Mon, 12 Sep 2022 12:58:28 +0200 | From | Borislav Petkov <> | Subject | Re: Performance Regression in Linux Kernel 5.19 |
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A couple more notes after talking to tglx:
So this works as expected. The threat model where the guest needs to protect itself from malicious userspace is there so if the guest emulates a CPU which is affected by retbleed and the hypervisor exposes SPEC_CTRL, then the guest *should* enable IBRS to flush the RSB.
It is a lot nastier if the guest emulates a CPU which is *not* affected by retbleed but the host uarch is - then the guest will be vulnerable and it would not even warn about it! So people should be careful what they do there.
In addition, if the guest trusts its userspace, it might disable IBRS in order not to suffer the penalty but that's left to the guest owner. The default setting has to be secure.
HTH.
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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