# Commit 8441fa806a3b778867867cd0159fa1722e90397e # Date 2024-02-27 14:58:20 +0100 # Author Roger Pau Monné # Committer Jan Beulich x86/spec: fix INDIRECT_THUNK option to only be set when build-enabled Attempt to provide a more helpful error message when the user attempts to set spec-ctrl=bti-thunk option but the support is build-time disabled. While there also adjust the command line documentation to mention CONFIG_INDIRECT_THUNK instead of INDIRECT_THUNK. Reported-by: Andrew Cooper Signed-off-by: Roger Pau Monné Reviewed-by: Jan Beulich --- a/docs/misc/xen-command-line.pandoc +++ b/docs/misc/xen-command-line.pandoc @@ -2033,11 +2033,11 @@ guests to use. performance reasons dom0 is unprotected by default. If it is necessary to protect dom0 too, boot with `spec-ctrl=ibpb-entry`. -If Xen was compiled with INDIRECT_THUNK support, `bti-thunk=` can be used to -select which of the thunks gets patched into the `__x86_indirect_thunk_%reg` -locations. The default thunk is `retpoline` (generally preferred), with the -alternatives being `jmp` (a `jmp *%reg` gadget, minimal overhead), and -`lfence` (an `lfence; jmp *%reg` gadget). +If Xen was compiled with `CONFIG_INDIRECT_THUNK` support, `bti-thunk=` can be +used to select which of the thunks gets patched into the +`__x86_indirect_thunk_%reg` locations. The default thunk is `retpoline` +(generally preferred), with the alternatives being `jmp` (a `jmp *%reg` gadget, +minimal overhead), and `lfence` (an `lfence; jmp *%reg` gadget). On hardware supporting IBRS (Indirect Branch Restricted Speculation), the `ibrs=` option can be used to force or prevent Xen using the feature itself. --- a/xen/arch/x86/spec_ctrl.c +++ b/xen/arch/x86/spec_ctrl.c @@ -251,7 +251,8 @@ static int __init parse_spec_ctrl(const } /* Xen's speculative sidechannel mitigation settings. */ - else if ( !strncmp(s, "bti-thunk=", 10) ) + else if ( IS_ENABLED(CONFIG_INDIRECT_THUNK) && + !strncmp(s, "bti-thunk=", 10) ) { s += 10;