xref: /openbmc/linux/arch/x86/hyperv/hv_spinlock.c (revision 28b82352)
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 
19 static bool __initdata hv_pvspin = true;
20 
hv_qlock_kick(int cpu)21 static void hv_qlock_kick(int cpu)
22 {
23 	__apic_send_IPI(cpu, X86_PLATFORM_IPI_VECTOR);
24 }
25 
hv_qlock_wait(u8 * byte,u8 val)26 static void hv_qlock_wait(u8 *byte, u8 val)
27 {
28 	unsigned long flags;
29 
30 	if (in_nmi())
31 		return;
32 
33 	/*
34 	 * Reading HV_X64_MSR_GUEST_IDLE MSR tells the hypervisor that the
35 	 * vCPU can be put into 'idle' state. This 'idle' state is
36 	 * terminated by an IPI, usually from hv_qlock_kick(), even if
37 	 * interrupts are disabled on the vCPU.
38 	 *
39 	 * To prevent a race against the unlock path it is required to
40 	 * disable interrupts before accessing the HV_X64_MSR_GUEST_IDLE
41 	 * MSR. Otherwise, if the IPI from hv_qlock_kick() arrives between
42 	 * the lock value check and the rdmsrl() then the vCPU might be put
43 	 * into 'idle' state by the hypervisor and kept in that state for
44 	 * an unspecified amount of time.
45 	 */
46 	local_irq_save(flags);
47 	/*
48 	 * Only issue the rdmsrl() when the lock state has not changed.
49 	 */
50 	if (READ_ONCE(*byte) == val) {
51 		unsigned long msr_val;
52 
53 		rdmsrl(HV_X64_MSR_GUEST_IDLE, msr_val);
54 
55 		(void)msr_val;
56 	}
57 	local_irq_restore(flags);
58 }
59 
60 /*
61  * Hyper-V does not support this so far.
62  */
hv_vcpu_is_preempted(int vcpu)63 __visible bool hv_vcpu_is_preempted(int vcpu)
64 {
65 	return false;
66 }
67 PV_CALLEE_SAVE_REGS_THUNK(hv_vcpu_is_preempted);
68 
hv_init_spinlocks(void)69 void __init hv_init_spinlocks(void)
70 {
71 	if (!hv_pvspin || !apic ||
72 	    !(ms_hyperv.hints & HV_X64_CLUSTER_IPI_RECOMMENDED) ||
73 	    !(ms_hyperv.features & HV_MSR_GUEST_IDLE_AVAILABLE)) {
74 		pr_info("PV spinlocks disabled\n");
75 		return;
76 	}
77 	pr_info("PV spinlocks enabled\n");
78 
79 	__pv_init_lock_hash();
80 	pv_ops.lock.queued_spin_lock_slowpath = __pv_queued_spin_lock_slowpath;
81 	pv_ops.lock.queued_spin_unlock = PV_CALLEE_SAVE(__pv_queued_spin_unlock);
82 	pv_ops.lock.wait = hv_qlock_wait;
83 	pv_ops.lock.kick = hv_qlock_kick;
84 	pv_ops.lock.vcpu_is_preempted = PV_CALLEE_SAVE(hv_vcpu_is_preempted);
85 }
86 
hv_parse_nopvspin(char * arg)87 static __init int hv_parse_nopvspin(char *arg)
88 {
89 	hv_pvspin = false;
90 	return 0;
91 }
92 early_param("hv_nopvspin", hv_parse_nopvspin);
93