Disable Spectre and Meltdown mitigations

Question

Can I disable Spectre and Meltdown mitigation features in Ubuntu 18.04LTS?

I want to test how much more performance I gain when I disable these two features in Linux, and if the performance is big, to make it permanently.

Answer 1

A number of kernel boot parameters are available to disable or fine-tune hardware vulnerability mitigations:

• for Spectre v1 and v2: nospectre_v1 (x86, PowerPC), nospectre_v2 (x86, PowerPC, S/390, ARM64), spectre_v2_user=off (x86)
• for SSB: spec_store_bypass_disable=off (x86, PowerPC), ssbd=force-off (ARM64)
• for L1TF: l1tf=off (x86)
• for MDS: mds=off (x86)
• for TAA: tsx_async_abort=off
• for iTLB multihit: kvm.nx_huge_pages=off
• for SRBDS: srbds=off
• for retbleed: retbleed=off
• KPTI can be disabled with nopti (x86, PowerPC) or kpti=0 (ARM64)

A meta-parameter, mitigations, was introduced in 5.2 and back-ported to 5.1.2, 5.0.16, and 4.19.43 (and perhaps others). It can be used to control all mitigations, on all architectures, as follows:

• mitigations=off will disable all optional CPU mitigations;
• mitigations=auto (the default setting) will mitigate all known CPU vulnerabilities, but leave SMT enabled (if it is already);
• mitigations=auto,nosmt will mitigate all known CPU vulnerabilities and disable SMT if appropriate.

Some of these can be toggled at runtime; see the linked documentation for details.

Answer 2

With a kernel 5.1.13 or newer :

On boot parameter you can use

mitigations=off 

With a kernel older than 5.1.13 :

noibrs noibpb nopti nospectre_v2 nospectre_v1 l1tf=off nospec_store_bypass_disable no_stf_barrier mds=off mitigations=off 

Add either mitigations=off or that long one-liner to your /etc/sysconfig/grub and re-generate grub's configuration file with

grub2-mkconfig

(your distributions procedure will vary).

Debian/Ubuntu derived distributions:

Edit the file /etc/default/grub then run

update-grub

Answer 3

On Fedora 37, with a sufficiently new kernel, mitigations status can be displayed by printing the content of files under /sys/devices/system/cpu/vulnerabilities/.

$ grep . /sys/devices/system/cpu/vulnerabilities/*
/sys/devices/system/cpu/vulnerabilities/itlb_multihit:KVM: Mitigation: VMX disabled
/sys/devices/system/cpu/vulnerabilities/l1tf:Not affected
/sys/devices/system/cpu/vulnerabilities/mds:Not affected
/sys/devices/system/cpu/vulnerabilities/meltdown:Not affected
/sys/devices/system/cpu/vulnerabilities/mmio_stale_data:Mitigation: Clear CPU buffers; SMT vulnerable
/sys/devices/system/cpu/vulnerabilities/retbleed:Mitigation: Enhanced IBRS
/sys/devices/system/cpu/vulnerabilities/spec_store_bypass:Mitigation: Speculative Store Bypass disabled via prctl
/sys/devices/system/cpu/vulnerabilities/spectre_v1:Mitigation: usercopy/swapgs barriers and __user pointer sanitization
/sys/devices/system/cpu/vulnerabilities/spectre_v2:Mitigation: Enhanced IBRS, IBPB: conditional, RSB filling, PBRSB-eIBRS: SW sequence
/sys/devices/system/cpu/vulnerabilities/srbds:Mitigation: Microcode
/sys/devices/system/cpu/vulnerabilities/tsx_async_abort:Not affected

To disable the mitigations in one swoop, do

sudo grubby --update-kernel=ALL --args="mitigations=off"

and behold how the printout changed after a reboot

$ grep . /sys/devices/system/cpu/vulnerabilities/*
/sys/devices/system/cpu/vulnerabilities/itlb_multihit:KVM: Mitigation: VMX disabled
/sys/devices/system/cpu/vulnerabilities/l1tf:Not affected
/sys/devices/system/cpu/vulnerabilities/mds:Not affected
/sys/devices/system/cpu/vulnerabilities/meltdown:Not affected
/sys/devices/system/cpu/vulnerabilities/mmio_stale_data:Vulnerable
/sys/devices/system/cpu/vulnerabilities/retbleed:Vulnerable
/sys/devices/system/cpu/vulnerabilities/spec_store_bypass:Vulnerable
/sys/devices/system/cpu/vulnerabilities/spectre_v1:Vulnerable: __user pointer sanitization and usercopy barriers only; no swapgs barriers
/sys/devices/system/cpu/vulnerabilities/spectre_v2:Vulnerable, IBPB: disabled, STIBP: disabled, PBRSB-eIBRS: Vulnerable
/sys/devices/system/cpu/vulnerabilities/srbds:Vulnerable
/sys/devices/system/cpu/vulnerabilities/tsx_async_abort:Not affected

Beware: Spectre proof-of-concept exploits can run in browser JavaScript

As the leaky.page proof-of-concept demonstrates, this vulnerability can be exploited from browser JavaScript code. Therefore, as long as you use a web browser, there is value in keeping the mitigations on.