4.9-stable review patch. If anyone has any objections, please let me know.
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From: Andy Lutomirski luto@kernel.org
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) 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/f1e7d965998018450a7a70c2823873686a8b21c0.150752474... Cc: Hugh Dickins hughd@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- 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 */