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 <andrew.coop...@citrix.com>
Signed-off-by: Roger Pau Monné <roger....@citrix.com>
---
Changes since v2:
- Adjust documentation.
- Use IS_ENABLED() instead of #ifdef.
Changes since v1:
- New in this version.
---
docs/misc/xen-command-line.pandoc | 10 +++++-----
xen/arch/x86/spec_ctrl.c | 7 ++++++-
2 files changed, 11 insertions(+), 6 deletions(-)
diff --git a/docs/misc/xen-command-line.pandoc
b/docs/misc/xen-command-line.pandoc
index be76be8d5365..02896598df6f 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -2417,11 +2417,11 @@ 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.
diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index 5fae80774519..ca82b9e41ccd 100644
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -241,7 +241,12 @@ static int __init cf_check parse_spec_ctrl(const char *s)
{
s += 10;
- if ( !cmdline_strcmp(s, "retpoline") )
+ if ( !IS_ENABLED(CONFIG_INDIRECT_THUNK) )
+ {
+ no_config_param("INDIRECT_THUNK", "spec-ctrl=bti-thunk", s,
ss);
+ rc = -EINVAL;
+ }
+ else if ( !cmdline_strcmp(s, "retpoline") )
opt_thunk = THUNK_RETPOLINE;
else if ( !cmdline_strcmp(s, "lfence") )
opt_thunk = THUNK_LFENCE;
--
2.43.0
