[PATCH 2/2] x86/kvm/vmx: Fix VMX vs hrtimer_rearm_deferred()

[Peter Zijlstra <peterz@infradead.org>]

Vishal reported that KVM unit test 'x2apic' started failing after commit
0e98eb14814e ("entry: Prepare for deferred hrtimer rearming").

The reason is that KVM/VMX is injecting interrupts while it has IRQs disabled,
for a context that will enable IRQs, this means that regs->flags.X86_EFLAGS_IF
== 0 and the irqentry_exit() will not DTRT.

Notably, irqentry_exit() must not call hrtimer_rearm_deferred() when the return
context does not have IF set, because this will cause problems vs NMIs.

Therefore, fix up the state after the injection.

Fixes: 0e98eb14814e ("entry: Prepare for deferred hrtimer rearming")
Reported-by: "Verma, Vishal L" <vishal.l.verma@intel.com>
Closes: https://lore.kernel.org/r/70cd3e97fbb796e2eb2ff8cd4b7614ada05a5f24.camel%40intel.com
Suggested-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: "Verma, Vishal L" <vishal.l.verma@intel.com>
---
 arch/x86/entry/common.c |   13 +++++++++++++
 1 file changed, 13 insertions(+)

--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -2,6 +2,7 @@
 
 #include <linux/entry-common.h>
 #include <linux/kvm_types.h>
+#include <linux/hrtimer_rearm.h>
 #include <asm/fred.h>
 #include <asm/desc.h>
 
@@ -27,6 +28,18 @@ noinstr void x86_entry_from_kvm(unsigned
 #else
 		idt_entry_from_kvm(vector);
 #endif
+		/*
+		 * Strictly speaking, only the NMI path requires noinstr.
+		 */
+		instrumentation_begin();
+		/*
+		 * KVM/VMX will dispatch from IRQ-disabled but for a context
+		 * that will have IRQs-enabled. This confuses the entry code
+		 * and it will not have reprogrammed the timer. Do so now.
+		 */
+		hrtimer_rearm_deferred();
+		instrumentation_end();
+
 		return;
 	}