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    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@suse.de>
    AuthorDate: Wed, 18 Jul 2018 11:41:15 +0200
    Committer: Thomas Gleixner <tglx@linutronix.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@kernel.org>
    Signed-off-by: Joerg Roedel <jroedel@suse.de>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Tested-by: Pavel Machek <pavel@ucw.cz>
    Cc: "H . Peter Anvin" <hpa@zytor.com>
    Cc: linux-mm@kvack.org
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Dave Hansen <dave.hansen@intel.com>
    Cc: Josh Poimboeuf <jpoimboe@redhat.com>
    Cc: Juergen Gross <jgross@suse.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Borislav Petkov <bp@alien8.de>
    Cc: Jiri Kosina <jkosina@suse.cz>
    Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
    Cc: Brian Gerst <brgerst@gmail.com>
    Cc: David Laight <David.Laight@aculab.com>
    Cc: Denys Vlasenko <dvlasenk@redhat.com>
    Cc: Eduardo Valentin <eduval@amazon.com>
    Cc: Greg KH <gregkh@linuxfoundation.org>
    Cc: Will Deacon <will.deacon@arm.com>
    Cc: aliguori@amazon.com
    Cc: daniel.gruss@iaik.tugraz.at
    Cc: hughd@google.com
    Cc: keescook@google.com
    Cc: Andrea Arcangeli <aarcange@redhat.com>
    Cc: Waiman Long <llong@redhat.com>
    Cc: "David H . Gutteridge" <dhgutteridge@sympatico.ca>
    Cc: joro@8bytes.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: */
    \
     
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