hv_spinlock.c

 1// SPDX-License-Identifier: GPL-2.0
 2
 3/*
 4 * Hyper-V specific spinlock code.
 5 *
 6 * Copyright (C) 2018, Intel, Inc.
 7 *
 8 * Author : Yi Sun <yi.y.sun@intel.com>
 9 */
10
11#define pr_fmt(fmt) "Hyper-V: " fmt
12
13#include <linux/spinlock.h>
14
15#include <asm/mshyperv.h>
16#include <asm/paravirt.h>
17#include <asm/apic.h>
18#include <asm/msr.h>
19
20static bool hv_pvspin __initdata = true;
21
22static void hv_qlock_kick(int cpu)
23{
24	__apic_send_IPI(cpu, X86_PLATFORM_IPI_VECTOR);
25}
26
27static void hv_qlock_wait(u8 *byte, u8 val)
28{
29	unsigned long flags;
30
31	if (in_nmi())
32		return;
33
34	/*
35	 * Reading HV_X64_MSR_GUEST_IDLE MSR tells the hypervisor that the
36	 * vCPU can be put into 'idle' state. This 'idle' state is
37	 * terminated by an IPI, usually from hv_qlock_kick(), even if
38	 * interrupts are disabled on the vCPU.
39	 *
40	 * To prevent a race against the unlock path it is required to
41	 * disable interrupts before accessing the HV_X64_MSR_GUEST_IDLE
42	 * MSR. Otherwise, if the IPI from hv_qlock_kick() arrives between
43	 * the lock value check and the rdmsrq() then the vCPU might be put
44	 * into 'idle' state by the hypervisor and kept in that state for
45	 * an unspecified amount of time.
46	 */
47	local_irq_save(flags);
48	/*
49	 * Only issue the rdmsrq() when the lock state has not changed.
50	 */
51	if (READ_ONCE(*byte) == val) {
52		unsigned long msr_val;
53
54		rdmsrq(HV_X64_MSR_GUEST_IDLE, msr_val);
55
56		(void)msr_val;
57	}
58	local_irq_restore(flags);
59}
60
61/*
62 * Hyper-V does not support this so far.
63 */
64__visible bool hv_vcpu_is_preempted(int vcpu)
65{
66	return false;
67}
68
69PV_CALLEE_SAVE_REGS_THUNK(hv_vcpu_is_preempted);
70
71void __init hv_init_spinlocks(void)
72{
73	if (!hv_pvspin || !apic ||
74	    !(ms_hyperv.hints & HV_X64_CLUSTER_IPI_RECOMMENDED) ||
75	    !(ms_hyperv.features & HV_MSR_GUEST_IDLE_AVAILABLE)) {
76		pr_info("PV spinlocks disabled\n");
77		return;
78	}
79	pr_info("PV spinlocks enabled\n");
80
81	__pv_init_lock_hash();
82	pv_ops.lock.queued_spin_lock_slowpath = __pv_queued_spin_lock_slowpath;
83	pv_ops.lock.queued_spin_unlock = PV_CALLEE_SAVE(__pv_queued_spin_unlock);
84	pv_ops.lock.wait = hv_qlock_wait;
85	pv_ops.lock.kick = hv_qlock_kick;
86	pv_ops.lock.vcpu_is_preempted = PV_CALLEE_SAVE(hv_vcpu_is_preempted);
87}
88
89static __init int hv_parse_nopvspin(char *arg)
90{
91	hv_pvspin = false;
92	return 0;
93}
94early_param("hv_nopvspin", hv_parse_nopvspin);