[PATCH] x86/bugs: Switch to "auto" when "ibrs" selected on Enhanced IBRS parts

[Pawan Gupta]

Currently spectre_v2=ibrs forces write to MSR_IA32_SPEC_CTRL at every
entry and exit. On Enhanced IBRS parts setting MSR_IA32_SPEC_CTRL[IBRS]
only once at bootup is sufficient. MSR write at every kernel entry/exit
incur unnecessary penalty that can be avoided.

When Enhanced IBRS feature is present, switch from "ibrs" to "auto" mode
so that appropriate mitigation is selected.

Fixes: 7c693f54c873 ("x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS")
Cc: stable@xxxxxxxxxxxxxxx # 5.10+
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
---
 arch/x86/kernel/cpu/bugs.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 0dd04713434b..7d7ebfdfbeda 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1303,6 +1303,12 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
 		return SPECTRE_V2_CMD_AUTO;
 	}
 
+	if (cmd == SPECTRE_V2_CMD_IBRS && boot_cpu_has(X86_FEATURE_IBRS_ENHANCED)) {
+		pr_err("%s selected but CPU supports Enhanced IBRS. Switching to AUTO select\n",
+		       mitigation_options[i].option);
+		return SPECTRE_V2_CMD_AUTO;
+	}
+
 	spec_v2_print_cond(mitigation_options[i].option,
 			   mitigation_options[i].secure);
 	return cmd;

base-commit: 72a8e05d4f66b5af7854df4490e3135168694b6b
-- 
2.35.3