4.9-stable review patch. If anyone has any objections, please let me know.
------------------ From: Andy Lutomirski <[email protected]> commit 924c6b900cfdf376b07bccfd80e62b21914f8a5a upstream. 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) <[email protected]> Signed-off-by: Andy Lutomirski <[email protected]> Signed-off-by: Thomas Gleixner <[email protected]> Cc: Borislav Petkov <[email protected]> Link: https://lkml.kernel.org/r/f1e7d965998018450a7a70c2823873686a8b21c0.1507524746.git.l...@kernel.org Cc: Hugh Dickins <[email protected]> Signed-off-by: Greg Kroah-Hartman <[email protected]> --- arch/x86/kernel/reboot.c | 4 ++++ 1 file changed, 4 insertions(+) --- a/arch/x86/kernel/reboot.c +++ b/arch/x86/kernel/reboot.c @@ -106,6 +106,10 @@ void __noreturn machine_real_restart(uns 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 */
