KVM: x86/xen: Mark poll_evtchn accesses with READ_ONCE()/WRITE_ONCE()

Use READ_ONCE() and WRITE_ONCE() for runtime accesses to poll_evtchn.
This marks the intentionally concurrent scalar accesses and prevents the
compiler from splitting, merging, or inventing accesses.

kvm_xen_schedop_poll() publishes the single port, or -1 for multiple
ports, before setting poll_mask and halting the vCPU.  Event delivery can
call kvm_xen_check_poller() on another CPU while the vCPU thread publishes
that value or resets the field to zero after returning from
kvm_vcpu_halt():

  vCPU thread                         event delivery thread
  -----------                         ---------------------
  poll_evtchn = port
  set_bit(poll_mask)
  kvm_vcpu_halt()
                                      poll_evtchn = READ
  poll_evtchn = 0
  clear_bit(poll_mask)

The plain read and writes therefore race.  KCSAN reported:

  BUG: KCSAN: data-race in kvm_xen_hypercall / kvm_xen_set_evtchn_fast

  read to 0xffff888112f55af0 of 4 bytes by task 98:
   kvm_xen_set_evtchn_fast+0x204/0x7c0
   kvm_xen_hvm_evtchn_send+0xab/0x100
   kvm_arch_vm_ioctl+0xb31/0xd90
   kvm_vm_ioctl+0xf42/0x16c0

  write to 0xffff888112f55af0 of 4 bytes by task 96:
   kvm_xen_hypercall+0xd8d/0xf50
   kvm_emulate_hypercall+0x157/0x1d0
   vmx_handle_exit+0x40f/0xae0
   vcpu_run+0x137f/0x27d0
   kvm_arch_vcpu_ioctl_run+0x5a5/0x970

The field is an aligned int on x86.  Access annotations preserve the
existing matching, callback, and poll-mask control flow while making the
single-copy access requirement explicit.

Fixes: 1a65105a5aba ("KVM: x86/xen: handle PV spinlocks slowpath")
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
Link: https://patch.msgid.link/20260831213632.81023-12-dwmw2@infradead.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

0 files changed, 0 insertions, 0 deletions