KVM: x86/xen: ignore the VCPU_SSHOTTMR_future flag

Upstream Xen now ignores _VCPU_SSHOTTMR_future[1], since the only guest
kernel ever to use it was buggy. By ignoring the flag the guest will
always get a callback if it sets a negative timeout which upstream Xen
has determined not to cause problems for any guest setting the flag.

[1] https://xenbits.xen.org/gitweb/?p=xen.git;a=commitdiff;h=19c6cbd909

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Link: https://lore.kernel.org/r/20231004174628.2073263-1-paul@xen.org
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: Paul Durrant

arch/x86/kvm/xen.c

@@ -1423,12 +1423,8 @@ static bool kvm_xen_hcall_vcpu_op(struct kvm_vcpu *vcpu, bool longmode, int cmd,
 			return true;
 		}
 
+		/* A delta <= 0 results in an immediate callback, which is what we want */
 		delta = oneshot.timeout_abs_ns - get_kvmclock_ns(vcpu->kvm);
-		if ((oneshot.flags & VCPU_SSHOTTMR_future) && delta < 0) {
-			*r = -ETIME;
-			return true;
-		}
-
 		kvm_xen_start_timer(vcpu, oneshot.timeout_abs_ns, delta);
 		*r = 0;
 		return true;