[PATCH] smp/hotplug, x86/vmware: Put offline vCPUs in halt instead of mwait

[Srivatsa S. Bhat]

From: Srivatsa S. Bhat (VMware) <srivatsa@xxxxxxxxxxxxx>

VMware ESXi allows enabling a passthru mwait CPU-idle state in the
guest using the following VMX option:

monitor_control.mwait_in_guest = "TRUE"

This lets a vCPU in mwait to remain in guest context (instead of
yielding to the hypervisor via a VMEXIT), which helps speed up
wakeups from idle.

However, this runs into problems with CPU hotplug, because the Linux
CPU offline path prefers to put the vCPU-to-be-offlined in mwait
state, whenever mwait is available. As a result, since a vCPU in mwait
remains in guest context and does not yield to the hypervisor, an
offline vCPU *appears* to be 100% busy as viewed from ESXi, which
prevents the hypervisor from running other vCPUs or workloads on the
corresponding pCPU (particularly when vCPU - pCPU mappings are
statically defined by the user). [ Note that such a vCPU is not
actually busy spinning though; it remains in mwait idle state in the
guest ].

Fix this by overriding the CPU offline play_dead() callback for VMware
hypervisor, by putting the CPU in halt state (which actually yields to
the hypervisor), even if mwait support is available.

Signed-off-by: Srivatsa S. Bhat (VMware) <srivatsa@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Alexey Makhalov <amakhalov@xxxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: VMware PV-Drivers Reviewers <pv-drivers@xxxxxxxxxx>
---

 arch/x86/kernel/cpu/vmware.c |   17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/arch/x86/kernel/cpu/vmware.c b/arch/x86/kernel/cpu/vmware.c
index c04b933f48d3..420e359ed9bb 100644
--- a/arch/x86/kernel/cpu/vmware.c
+++ b/arch/x86/kernel/cpu/vmware.c
@@ -27,6 +27,7 @@
 #include <linux/clocksource.h>
 #include <linux/cpu.h>
 #include <linux/reboot.h>
+#include <linux/tboot.h>
 #include <linux/static_call.h>
 #include <asm/div64.h>
 #include <asm/x86_init.h>
@@ -312,6 +313,21 @@ static int vmware_cpu_down_prepare(unsigned int cpu)
 	local_irq_enable();
 	return 0;
 }
+
+static void vmware_play_dead(void)
+{
+	play_dead_common();
+	tboot_shutdown(TB_SHUTDOWN_WFS);
+
+	/*
+	 * Put the vCPU going offline in halt instead of mwait (even
+	 * if mwait support is available), to make sure that the
+	 * offline vCPU yields to the hypervisor (which may not happen
+	 * with mwait, for example, if the guest's VMX is configured
+	 * to retain the vCPU in guest context upon mwait).
+	 */
+	hlt_play_dead();
+}
 #endif
 
 static __init int activate_jump_labels(void)
@@ -349,6 +365,7 @@ static void __init vmware_paravirt_ops_setup(void)
 #ifdef CONFIG_SMP
 		smp_ops.smp_prepare_boot_cpu =
 			vmware_smp_prepare_boot_cpu;
+		smp_ops.play_dead = vmware_play_dead;
 		if (cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
 					      "x86/vmware:online",
 					      vmware_cpu_online,