[PATCH v4 08/10] x86/mm: Add nopcid to turn off PCID

[Andy Lutomirski]

The parameter is only present on x86_64 systems to save a few bytes,
as PCID is always disabled on x86_32.

Reviewed-by: Nadav Amit <nadav.a...@gmail.com>
Reviewed-by: Borislav Petkov <b...@suse.de>
Reviewed-by: Thomas Gleixner <t...@linutronix.de>
Signed-off-by: Andy Lutomirski <l...@kernel.org>
---
 Documentation/admin-guide/kernel-parameters.txt |  2 ++
 arch/x86/kernel/cpu/common.c                    | 18 ++++++++++++++++++
 2 files changed, 20 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt 
b/Documentation/admin-guide/kernel-parameters.txt
index 7737ab5d04b2..705666a44ba2 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2648,6 +2648,8 @@
        nopat           [X86] Disable PAT (page attribute table extension of
                        pagetables) support.
 
+       nopcid          [X86-64] Disable the PCID cpu feature.
+
        norandmaps      Don't use address space randomization.  Equivalent to
                        echo 0 > /proc/sys/kernel/randomize_va_space
 
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index c8b39870f33e..904485e7b230 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -168,6 +168,24 @@ static int __init x86_mpx_setup(char *s)
 }
 __setup("nompx", x86_mpx_setup);
 
+#ifdef CONFIG_X86_64
+static int __init x86_pcid_setup(char *s)
+{
+       /* require an exact match without trailing characters */
+       if (strlen(s))
+               return 0;
+
+       /* do not emit a message if the feature is not present */
+       if (!boot_cpu_has(X86_FEATURE_PCID))
+               return 1;
+
+       setup_clear_cpu_cap(X86_FEATURE_PCID);
+       pr_info("nopcid: PCID feature disabled\n");
+       return 1;
+}
+__setup("nopcid", x86_pcid_setup);
+#endif
+
 static int __init x86_noinvpcid_setup(char *s)
 {
        /* noinvpcid doesn't accept parameters */
-- 
2.9.4