6.15-stable review patch. If anyone has any objections, please let me know.
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From: David Woodhouse dwmw@amazon.co.uk
commit a7f4dff21fd744d08fa956c243d2b1795f23cbf7 upstream.
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 dwmw@amazon.co.uk Reviewed-by: Paul Durrant paul@xen.org Link: https://lore.kernel.org/r/e489252745ac4b53f1f7f50570b03fb416aa2065.camel@inf... [sean: massage comment] Signed-off-by: Sean Christopherson seanjc@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/kvm/xen.c | 15 +++++++++++++-- 1 file changed, 13 insertions(+), 2 deletions(-)
--- a/arch/x86/kvm/xen.c +++ b/arch/x86/kvm/xen.c @@ -1970,8 +1970,19 @@ int kvm_xen_setup_evtchn(struct kvm *kvm { struct kvm_vcpu *vcpu;
- if (ue->u.xen_evtchn.port >= max_evtchn_port(kvm)) - return -EINVAL; + /* + * Don't check for the port being within range of max_evtchn_port(). + * Userspace can configure what ever targets it likes; events just won't + * be delivered if/while the target is invalid, just like userspace can + * configure MSIs which target non-existent APICs. + * + * This allow 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)