Maybe the grandparent is trying to refer to IOMMUs.

This. Your system will almost certainly have an IOMMU. But that can’t be said for all systems that the Linux kernel supports.

However your IOMMU may not actually be in use. It's not in use by default on Linux and on most Linux distros as it tends to break things on random hardware that isn't setup right. It tends to work most of the time on servers.

Ubuntu 22.04 tried to turn it on by default but switched it off again due to random mostly graphics related regressions on random hardware: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1971699

Some other history: https://www.phoronix.com/news/Intel-IOMMU-Gfx-Default-Try

We really do need it though. I am always reminded of the very old Apple "Firewire Memory Bypass" which rendered flames to the screen just by plugging a firewire device in - because firewire had direct and originally unprotected DMA access: https://www.pentestpartners.com/security-blog/hack-demo-vide...

It is for this reason that even without IOMMU, as a workaround, you have to often give permission to thunderbolt devices to connect. Some details on that here: https://wiki.archlinux.org/title/Thunderbolt

There is also a small but noticable performance hit to using the IOMMU, not so noticable on a general setup but if you are doing high-speed disk & network I/O like ceph storage in excess of 10Gbit/s or millions of IOPS you will notice it. You can Google that.

You can also run into other weird behaviour, for example when using kdump to create a kernel crash dump it will kexec from the old kernel into a new kernel to produce the crash dump. The system doesn't go through a firmwire/uefi/bios reset so the hardware state of network cards, etc, doesn't get reset. So if you have any hardware driver state that isn't properly reset, you might for example have your network card DMA a packet directly into host memory in the time window before it gets reset. With IOMMU that might trigger errors, with it off it will hopefully not overwrite anything important but may also overwrite something important :)

These things are all of course fixable, but since it's still off by default much of the time, lots of these bugs persist for a long time.

Disclaimer: I am not an expert in this area it's just anecodtes from my life as a Linux Geek & Support Engineer. Should be about 90% accurate but I am sure I glossed over some solid details :)