[patch] KVM: do VMXOFF upon reboot

[Ingo Molnar <mingo@elte.hu>]

* Ingo Molnar <mingo@elte.hu> wrote:

> Subject: [patch] call reboot notifier list when doing an emergency reboot
> From: Ingo Molnar <mingo@elte.hu>
> 
> my laptop (Lenovo T60) hangs during reboot if the shutdown notifiers are 
> not called. So the following command, which on other systems i use as a 
> quick way to reboot into a new kernel:
> 
>    echo b > /proc/sysrq-trigger
> 
> just hangs indefinitely after the kernel prints "Restarting system".

i also figured out which one of the many reboot notifiers makes the 
crutial difference: it's kvm_reboot().

So i think we should do the patch below - this makes reboot work even in 
atomic contexts. (My previous patch makes sense nevertheless, as reboot 
notifiers are quite useful in general, even during SysRq-b or panic 
reboots. For example the SATA disk caches are flushed.)

I have tested this without the other kernel/sys.c patch, and this solves 
the hung reboot problem equally well.

	Ingo

--------------------->
Subject: [patch] KVM: do VMXOFF upon reboot
From: Ingo Molnar <mingo@elte.hu>

my laptop's BIOS apparently gets confused if the kernel tries to
reboot without first turning VT context off, which results in a
hung (emergency-)reboot. So make sure this happens, right before
we reboot.

( NOTE: this is a dual-core system, but only the core where the
  BIOS executes seems to be affected - the other core can have an
  active VT context just fine - so we dont have to risk reboot
  robustness by doing a CPU cross-call in the emergency reboot
  handler. )

Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 arch/i386/kernel/reboot.c   |    8 ++++++++
 arch/x86_64/kernel/reboot.c |    7 +++++++
 2 files changed, 15 insertions(+)

Index: linux/arch/i386/kernel/reboot.c
===================================================================
--- linux.orig/arch/i386/kernel/reboot.c
+++ linux/arch/i386/kernel/reboot.c
@@ -318,6 +318,14 @@ void machine_shutdown(void)
 
 void machine_emergency_restart(void)
 {
+	unsigned long ecx = cpuid_ecx(1);
+
+	/*
+	 * Disable any possibly active VT context (if VT supported):
+	 */
+	if (test_bit(5, &ecx)) /* has VT support */
+		asm volatile (".byte 0x0f, 0x01, 0xc4"); /* vmxoff */
+
 	if (!reboot_thru_bios) {
 		if (efi_enabled) {
 			efi.reset_system(EFI_RESET_COLD, EFI_SUCCESS, 0, NULL);
Index: linux/arch/x86_64/kernel/reboot.c
===================================================================
--- linux.orig/arch/x86_64/kernel/reboot.c
+++ linux/arch/x86_64/kernel/reboot.c
@@ -114,8 +114,15 @@ void machine_shutdown(void)
 
 void machine_emergency_restart(void)
 {
+	unsigned long ecx = cpuid_ecx(1);
 	int i;
 
+	/*
+	 * Disable any possibly active VT context (if VT supported):
+	 */
+	if (test_bit(5, &ecx)) /* has VT support */
+		asm volatile (".byte 0x0f, 0x01, 0xc4"); /* vmxoff */
+
 	/* Tell the BIOS if we want cold or warm reboot */
 	*((unsigned short *)__va(0x472)) = reboot_mode;