From dc0bde86ef4be27670b3a3124dab5090ba608c68 Mon Sep 17 00:00:00 2001 From: David Woodhouse Date: Thu, 8 May 2025 21:19:47 +0100 Subject: [PATCH] KVM: x86/xen: Allow 'out of range' event channel ports in IRQ routing table. To avoid imposing an ordering constraint on userspace, allow 'invalid' event channel targets to be configured in the IRQ routing table. This is the same as accepting interrupts targeted at vCPUs which don't exist yet, which is already the case for both Xen event channels *and* for MSIs (which don't do any filtering of permitted APIC ID targets at all). If userspace actually *triggers* an IRQ with an invalid target, that will fail cleanly, as kvm_xen_set_evtchn_fast() also does the same range check. If KVM enforced that the IRQ target must be valid at the time it is *configured*, that would force userspace to create all vCPUs and do various other parts of setup (in this case, setting the Xen long_mode) before restoring the IRQ table. Cc: stable@vger.kernel.org Signed-off-by: David Woodhouse --- arch/x86/kvm/xen.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c index 38b33cdd4232d..441b4fe206f58 100644 --- a/arch/x86/kvm/xen.c +++ b/arch/x86/kvm/xen.c @@ -1970,8 +1970,18 @@ int kvm_xen_setup_evtchn(struct kvm *kvm, { struct kvm_vcpu *vcpu; - if (ue->u.xen_evtchn.port >= max_evtchn_port(kvm)) - return -EINVAL; + /* + * No check for being within range of max_evtchn_port() here. + * Userspace can configure what ever targets it likes; they + * just won't be delivered if/while they're invalid. Just like + * userspace can configure MSIs which target non-existent APICs. + * This means that on Live Migration and Live Update, the IRQ + * routing table can be restored *independently* of other things + * like creating vCPUs without imposing an ordering dependency on + * userspace. In this particular case, the problematic ordering + * would be with setting the Xen 'long mode' flag which changes + * max_evtchn_port() to allow 4096 instead of 1024 event channels. + */ /* We only support 2 level event channels for now */ if (ue->u.xen_evtchn.priority != KVM_IRQ_ROUTING_XEN_EVTCHN_PRIO_2LEVEL) -- 2.50.1