Explicitly invalidate KVM's internal Xen vCPU ID during vCPU creation instead of *trying* to set the Xen ID to the vCPU index by default, and forward singleshot timer hypercalls to userspace if the VMM hasn't set the Xen ID via KVM_XEN_VCPU_ATTR_TYPE_VCPU_ID. Using the vCPU's index as its default Xen ID is reasonable in concept, but in practice is horribly flawed as the index is left as '0' until after vCPU initialization completes, i.e. every vCPU gets a Xen ID of '0' by default. Forward hypercalls to userspace instead of trying to salvage any kind of default behavior, as all userspace implementations that support multiple vCPUs either don't enable the timer, are guaranteed to set Xen ID, or work only because *all* guests also screw up the singleshot timer hypercalls. The last scenarios is extremely unlikely given that Linux-as-a-guest uses the actual Xen vCPU ID when making timer hypercalls. In other words, for all intents and purposes, KVM's ABI is already that userspace must set the Xen vCPU ID, so just commit to that ABI. Note, KVM's handling of KVM_XEN_VCPU_ATTR_TYPE_VCPU_ID restricts the ID to KVM_MAX_VCPUS, so there's no chance of a valid ID colliding with -1u. Link: https://lore.kernel.org/all/20260612233017.1F9771F000E9@smtp.kernel.org Suggested-by: David Woodhouse Signed-off-by: Sean Christopherson --- arch/x86/kvm/xen.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c index 694b31c1fcc9..27c1aeeab8af 100644 --- a/arch/x86/kvm/xen.c +++ b/arch/x86/kvm/xen.c @@ -22,6 +22,7 @@ #include #include #include +#include #include #include @@ -1610,6 +1611,9 @@ static bool kvm_xen_hcall_vcpu_op(struct kvm_vcpu *vcpu, bool longmode, int cmd, if (!kvm_xen_timer_enabled(vcpu)) return false; + if (vcpu->arch.xen.vcpu_id == XEN_VCPU_ID_INVALID) + return false; + switch (cmd) { case VCPUOP_set_singleshot_timer: if (vcpu->arch.xen.vcpu_id != vcpu_id) { @@ -2299,7 +2303,7 @@ static bool kvm_xen_hcall_evtchn_send(struct kvm_vcpu *vcpu, u64 param, u64 *r) void kvm_xen_init_vcpu(struct kvm_vcpu *vcpu) { - vcpu->arch.xen.vcpu_id = vcpu->vcpu_idx; + vcpu->arch.xen.vcpu_id = XEN_VCPU_ID_INVALID; vcpu->arch.xen.poll_evtchn = 0; timer_setup(&vcpu->arch.xen.poll_timer, cancel_evtchn_poll, 0); -- 2.55.0.rc0.799.gd6f94ed593-goog