> Wonder what the defaults

See the Thunderclap FAQ, most of your questions are explained. Thunderclap is the DMA attack on Thunderbolt, and everything should be applicable to USB 4 as well, since USB 4 incorporated Thunderbolt directly.

https://thunderclap.io/

From the FAQ,

> In macOS 10.12.4 and later, Apple addressed the specific network card vulnerability we used to achieve a root shell. However the general scope of our work still applies; in particular that Thunderbolt devices have access to all network traffic and sometimes keystrokes and framebuffer data.

> Microsoft have enabled support for the IOMMU for Thunderbolt devices in Windows 10 version 1803, which shipped in 2018. Earlier hardware upgraded to 1803 requires a firmware update from the vendor. This brings them into line with the baseline for our work, however the more complex vulnerabilities we describe remain relevant.

> Recently, Intel have contributed patches to version 5.0 of the Linux kernel (shortly to be released) that enable the IOMMU for Thunderbolt and prevent the protection-bypass vulnerability that uses the ATS feature of PCI Express.

But note that Microsoft says on its website,

> This feature does not protect against DMA attacks via 1394/FireWire, PCMCIA, CardBus, ExpressCard, and so on.

Basically, IOMMU is enabled by default in most systems now. However, only on Thunderbolt devices (and USB 4 devices, since Thunderbolt is now its subset). But not on other PCI-E ports, such as the internal PCI-E ports, or external ones like 1394 or ExpressCard. I think it's due to fears of performance and compatibility issues, there's no reason why it cannot be implemented if one insists. Also, from the macOS example, you see that bad drivers can be a problem, even with IOMMU, if a driver is bad, attackers can exploit them and trick them to give more RAM access (If my memory is correct, Linux's implementation was better, but since it's an early FAQ, I don't know if macOS has improved).

> what the comparable parameters are for common consumer OSes.

On the Linux kernel, with an Intel CPU, the kernel option "intel_iommu=on" will enable IOMMU completely for everything, internal and external, which is the most ideal option. You should see

DMAR: IOMMU enabled

in dmesg. Some people report compatibility issues for Intel's integrated GPU that causes glitches or system hang, use "intel_iommu=on,igfx_off" should fix it (which is not really a security problem, the GPU is already in the CPU). I've been running my PC with IOMMU for years by now. didn't notice any problem for me.

IOMMU needs support from the CPU and motherboard chipset, Intel's IOMMU is called VT-D and marketed it as I/O virtualization for virtual machines. Most i3, i5, and i7 CPUs are supported. You may need to turn it on in BIOS if there's an option. It should work on most laptops, even an old Ivy Bridge laptop should work - which is good news, laptops are the most vulnerable platform. Unfortunately, on desktops, Intel intentionally removed IOMMU from high-end CPUs preferred by PC enthusiasts in the 1st (Nehalem), 2nd (Sandy Bridge), 3rd (Ivy Bridge), and 4th (Haswell) generations, possibly as a strategy to sell more Xeon chips to enterprise virtualization users (likely in line with their cripple-ECC strategy). Most high-end overclockable K-series CPUs have their IOMMU removed, while the non-K counterparts are supported. Also, in these generations, some motherboard chipsets are crippled and don't have IOMMU. Fortunately, Intel no longer does it since 5/6th gen (Broadwell/Skylake) CPUs.

No experience on AMD yet, but it should be similar. I think possibly better, my impression is that AMD doesn't intentional crippling IOMMU or ECC like Intel does (a nasty strategy...). Check the docs.