x86/mm/64: Fix reboot interaction with CR4.PCIDE

Trying to reboot via real mode fails with PCID on: long mode cannot
be exited while CR4.PCIDE is set.  (No, I have no idea why, but the
SDM and actual CPUs are in agreement here.)  The result is a GPF and
a hang instead of a reboot.

I didn't catch this in testing because neither my computer nor my VM
reboots this way.  I can trigger it with reboot=bios, though.

Fixes: 660da7c9228f ("x86/mm: Enable CR4.PCIDE on supported systems")
Reported-and-tested-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@alien8.de>
Link: https://lkml.kernel.org/r/f1e7d965998018450a7a70c2823873686a8b21c0.1507524746.git.luto@kernel.org

diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
index 54180fa..add33f6 100644 (file)
--- a/arch/x86/kernel/reboot.c
+++ b/arch/x86/kernel/reboot.c
@@ -105,6 +105,10 @@ void __noreturn machine_real_restart(unsigned int type)
        load_cr3(initial_page_table);
 #else
        write_cr3(real_mode_header->trampoline_pgd);
+
+       /* Exiting long mode will fail if CR4.PCIDE is set. */
+       if (static_cpu_has(X86_FEATURE_PCID))
+               cr4_clear_bits(X86_CR4_PCIDE);
 #endif
 
        /* Jump to the identity-mapped low memory code */