[PATCH 38/39] x86/mm/pti: Add Warning when booting on a PCID capable CPU

[Joerg Roedel]

From: Joerg Roedel <jroedel@xxxxxxx>

Warn the user in case the performance can be significantly
improved by switching to a 64-bit kernel.

Suggested-by: Andy Lutomirski <luto@xxxxxxxxxx>
Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
---
 arch/x86/mm/pti.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
index b879ccd..be8d2cd 100644
--- a/arch/x86/mm/pti.c
+++ b/arch/x86/mm/pti.c
@@ -517,6 +517,28 @@ void __init pti_init(void)
 
 	pr_info("enabled\n");
 
+#ifdef CONFIG_X86_32
+	/*
+	 * We check for X86_FEATURE_PCID here. But the init-code will
+	 * clear the feature flag on 32 bit because the feature is not
+	 * supported on 32 bit anyway. To print the warning we need to
+	 * check with cpuid directly again.
+	 */
+	if (cpuid_ecx(0x1) && BIT(17)) {
+		/* Use printk to work around pr_fmt() */
+		printk(KERN_WARNING "\n");
+		printk(KERN_WARNING "************************************************************\n");
+		printk(KERN_WARNING "** WARNING! WARNING! WARNING! WARNING! WARNING! WARNING!  **\n");
+		printk(KERN_WARNING "**                                                        **\n");
+		printk(KERN_WARNING "** You are using 32-bit PTI on a 64-bit PCID-capable CPU. **\n");
+		printk(KERN_WARNING "** Your performance will increase dramatically if you     **\n");
+		printk(KERN_WARNING "** switch to a 64-bit kernel!                             **\n");
+		printk(KERN_WARNING "**                                                        **\n");
+		printk(KERN_WARNING "** WARNING! WARNING! WARNING! WARNING! WARNING! WARNING!  **\n");
+		printk(KERN_WARNING "************************************************************\n");
+	}
+#endif
+
 	pti_clone_user_shared();
 
 	/* Undo all global bits from the init pagetables in head_64.S: */
-- 
2.7.4