linux-kernel - [tip:x86/pti] x86/mm/pti: Add Warning when booting on a PCID capable CPU

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Date:   Thu, 19 Jul 2018 16:38:21 -0700
From:   tip-bot for Joerg Roedel <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     gregkh@...uxfoundation.org, llong@...hat.com, bp@...en8.de,
        David.Laight@...lab.com, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, jgross@...e.com, jroedel@...e.de,
        aarcange@...hat.com, brgerst@...il.com, dvlasenk@...hat.com,
        jkosina@...e.cz, jpoimboe@...hat.com, hpa@...or.com,
        eduval@...zon.com, tglx@...utronix.de, dave.hansen@...el.com,
        mingo@...nel.org, will.deacon@....com, peterz@...radead.org,
        dhgutteridge@...patico.ca, pavel@....cz, luto@...nel.org,
        boris.ostrovsky@...cle.com
Subject: [tip:x86/pti] x86/mm/pti: Add Warning when booting on a PCID
 capable CPU

Commit-ID:  5e8105950a8b3e03e805299b4d05020ee4eda31a
Gitweb:     https://git.kernel.org/tip/5e8105950a8b3e03e805299b4d05020ee4eda31a
Author:     Joerg Roedel <jroedel@...e.de>
AuthorDate: Wed, 18 Jul 2018 11:41:15 +0200
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Fri, 20 Jul 2018 01:11:48 +0200

x86/mm/pti: Add Warning when booting on a PCID capable CPU

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

Suggested-by: Andy Lutomirski <luto@...nel.org>
Signed-off-by: Joerg Roedel <jroedel@...e.de>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Tested-by: Pavel Machek <pavel@....cz>
Cc: "H . Peter Anvin" <hpa@...or.com>
Cc: linux-mm@...ck.org
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Dave Hansen <dave.hansen@...el.com>
Cc: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: Juergen Gross <jgross@...e.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Jiri Kosina <jkosina@...e.cz>
Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc: Brian Gerst <brgerst@...il.com>
Cc: David Laight <David.Laight@...lab.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: Eduardo Valentin <eduval@...zon.com>
Cc: Greg KH <gregkh@...uxfoundation.org>
Cc: Will Deacon <will.deacon@....com>
Cc: aliguori@...zon.com
Cc: daniel.gruss@...k.tugraz.at
Cc: hughd@...gle.com
Cc: keescook@...gle.com
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: Waiman Long <llong@...hat.com>
Cc: "David H . Gutteridge" <dhgutteridge@...patico.ca>
Cc: joro@...tes.org
Link: https://lkml.kernel.org/r/1531906876-13451-39-git-send-email-joro@8bytes.org

---
 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 a536ecc91847..7b1c85759005 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: */