Patch "x86/mm/64: Fix reboot interaction with CR4.PCIDE" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

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

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:

     x86-mm-64-fix-reboot-interaction-with-cr4.pcide.patch

and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Andy Lutomirski <[email protected]>
Date: Sun, 8 Oct 2017 21:53:05 -0700
Subject: x86/mm/64: Fix reboot interaction with CR4.PCIDE

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.

---
 arch/x86/kernel/reboot.c |    4 ++++
 1 file changed, 4 insertions(+)

@@ -93,6 +93,10 @@ 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 */

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/x86-vm86-32-switch-to-flush_tlb_mm_range-in-mark_screen_rdonly.patch queue-4.4/x86-mm-add-the-nopcid-boot-option-to-turn-off-pcid.patch queue-4.4/x86-mm-enable-cr4.pcide-on-supported-systems.patch queue-4.4/x86-mm-remove-the-up-asm-tlbflush.h-code-always-use-the-formerly-smp-code.patch queue-4.4/x86-mm-reimplement-flush_tlb_page-using-flush_tlb_mm_range.patch queue-4.4/x86-mm-make-flush_tlb_mm_range-more-predictable.patch queue-4.4/x86-mm-remove-flush_tlb-and-flush_tlb_current_task.patch queue-4.4/x86-mm-disable-pcid-on-32-bit-kernels.patch queue-4.4/x86-mm-64-fix-reboot-interaction-with-cr4.pcide.patch