[PATCH] KVM: x86/xen: Use read_trylock() for GPC locks in hardirq paths

Hi KVM maintainers,

This patch fixes a lockdep WARNING in kvm_xen_set_evtchn_fast() when
called from xen_timer_callback() in hard IRQ context.

Bug report:
  https://syzkaller.appspot.com/bug?extid=919877893c9d28162dc2

Observed crash (unpatched):
  [ BUG: Invalid wait context ]
  (&gpc->lock) at kvm_xen_set_evtchn_fast+0x1fb
  context-{2:2}  (hard IRQ, from xen_timer_callback)

Fix:
  Use read_trylock() on gpc->lock and return -EWOULDBLOCK on failure.
  The existing slow path (timer_pending + kvm_xen_inject_timer_irqs)
  handles deferred delivery safely in vcpu_run context.

This patch is based on David Woodhouse's May 2026 series (patch 3/7).
I reproduced the syzbot C repro on x86_64 with nested KVM:

  Unpatched bzImage-b69053dd: 1x lockdep BUG at ~69s
  Patched bzImage-xen-fix:    0x lockdep BUG (same repro)

Tested-by: sai madhu <[email protected]>

Thanks,
sai madhu