linux-kernel - [PATCH v4 07/19] x86/reboot: Disable virtualization during reboot iff callback is registered

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Date:   Fri, 21 Jul 2023 13:18:47 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Kai Huang <kai.huang@...el.com>, Chao Gao <chao.gao@...el.com>
Subject: [PATCH v4 07/19] x86/reboot: Disable virtualization during reboot iff
 callback is registered

Attempt to disable virtualization during an emergency reboot if and only
if there is a registered virt callback, i.e. iff a hypervisor (KVM) is
active.  If there's no active hypervisor, then the CPU can't be operating
with VMX or SVM enabled (barring an egregious bug).

Checking for a valid callback instead of simply for SVM or VMX support
can also eliminates spurious NMIs by avoiding the unecessary call to
nmi_shootdown_cpus_on_restart().

Note, IRQs are disabled, which prevents KVM from coming along and
enabling virtualization after the fact.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kernel/reboot.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
index 85cb2dfcb67b..98e5db3fd7f4 100644
--- a/arch/x86/kernel/reboot.c
+++ b/arch/x86/kernel/reboot.c
@@ -22,7 +22,6 @@
 #include <asm/reboot_fixups.h>
 #include <asm/reboot.h>
 #include <asm/pci_x86.h>
-#include <asm/virtext.h>
 #include <asm/cpu.h>
 #include <asm/nmi.h>
 #include <asm/smp.h>
@@ -589,7 +588,7 @@ static void emergency_reboot_disable_virtualization(void)
 	 * Do the NMI shootdown even if virtualization is off on _this_ CPU, as
 	 * other CPUs may have virtualization enabled.
 	 */
-	if (cpu_has_vmx() || cpu_has_svm(NULL)) {
+	if (rcu_access_pointer(cpu_emergency_virt_callback)) {
 		/* Safely force _this_ CPU out of VMX/SVM operation. */
 		cpu_emergency_disable_virtualization();
 
-- 
2.41.0.487.g6d72f3e995-goog