[Xen-devel] CPU flags disappear on Spectre enabled host

[Olaf Hering <olaf@xxxxxxxxx>]

On a host that is booted with the following command line, certain cpu flags 
disappear in xen-4.9 and later:

(XEN) Command line: loglvl=all guest_loglvl=all console=com1 com1=57600 
cpuid=ibrsb,stibp,ibpb,ssbd spec-ctrl=ibrs,ibpb,ssbd,bti-thunk=retpoline 
xpti=yes

On my test system the difference in dom0s /proc/cpuinfo is like that:
--- xen-dom0-4.8-spectre.txt
+++ xen-dom0-4.12-spectre.txt
-arat
-arch_perfmon
-dtherm
-epb
-ida
+ss

The concern is that a domU started on a xen-4.8 or earlier host, which is then 
migrated to xen-4.9 or later may miss these cpu flags at runtime.

Is the loss of cpuflags intentional?

I just noticed that at least in staging a boot with "no-ibrsb no-stibp no-ibpb 
no-ssbd spec-ctrl=no-xen,rsb=no xpti=no" does still not show these flags in 
dom0.

Olaf

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