arch/x86/hyperv/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
  19 static bool __initdata hv_pvspin = true;
  20
  21 static void hv_qlock_kick(int cpu)
  22 {
  23         apic->send_IPI(cpu, X86_PLATFORM_IPI_VECTOR);
  24 }
  25
  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  */
  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
  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
  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);