Re: [PATCH v3 11/13] KVM: x86/xen: Mark poll_evtchn accesses with READ_ONCE()/WRITE_ONCE()

[Paul Durrant <xadimgnik@gmail.com>]

On 31/08/2026 22:26, David Woodhouse wrote:
> From: Chengfeng Ye <nicoyip.dev@gmail.com>
> 
> 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>
> ---
>   arch/x86/kvm/xen.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
> 
Reviewed-by: Paul Durrant <paul@xen.org>