my setup passes through a discrete PCI-Express graphics card to Windows running under KVM, and the card is restricted to address space allocated to the VM using the motherboard's IOMMU.
You are correct that it is dependent upon whether your motherboard has IOMMU [1]. Nevertheless, given the following paragraph, then it seems to be the reverse: it is generally unwise to willy-nilly give out PCI pass-through, unless you have confirmed that your hardware has IOMMU.
Xen documentation explicitly states how to determine whether you have IOMMU on your hardware.[2] The hardware listed here [3], while out of date, conspicuously notes, for example, that "Core i7...most "K" versions don't support VT-d", which, according to Intel [4], means that the i7-4770K, for example, does not support IOMMU (VT-d). This tells me that if you are concerned about this issue, then you must check your hardware, even if it is new hardware.
[4] the i7-4770K probably lacks IOMMU because it would negatively affect performance on a CPU that is sold as a near top-of-the-line CPU for gaming, which generally excludes most virtualization use cases.
my setup passes through a discrete PCI-Express graphics card to Windows running under KVM, and the card is restricted to address space allocated to the VM using the motherboard's IOMMU.