Retbleed mitigation slows Linux 5.19 VMs on ESXi by up to 70%

VMware’s performance engineers reported on the kernel mailing list that Linux 5.19 guests on ESXi run up to 70% slower in compute, 30% in networking and 13% in storage than on 5.18. They bisected the regression to commit 6ad0ad2bf8a6 (“x86/bugs: Report Intel retbleed vulnerability”), which makes the guest kernel use IBRS on their Skylake hosts to mitigate CVE-2022-29901, the Intel half of Retbleed. Kernel developer Peter Zijlstra’s reply, in full: “Well, duh.. :-)”. Booting with spectre_v2=off brings the speed back, and the vulnerability with it, as Tarnkappe notes (in German). Fast or safe, pick one.