Comment 19 for bug 1869006

@alex-l-williamson: is there any safe(ish) way to ignore RMRR coming from BIOS?

I don't know how IOMMU actually works in the kernel but theoretically if kernel had a flag forcing it to ignore certain RMRRs? If I understand this correctly ignoring an RMRR entry may cause two things:
1) DMA failure if remapping is attempted
2) If something (e.g. KVM) touches that region because we ignored RMRR the device memory may get corrupted

Linux already has mechanisms to ignore stubborn BIOSes (e.g. disabled x2APIC with no option to enable it in the BIOS).

The only thing I'm worried about is the thing you said:
> The more significant aspect when RMRRs are involved in this restriction is that an RMRR is
> essentially the platform firmware dictating that the host OS must maintain an identity map
> between the device and a range of physical address space. We don't know the purpose of that
> mapping, but we can assume that it allows the device to provide ongoing data for platform
> firmware to consume.

Does this mean that if a kernel is "blind" to a given RMRR region something else may break because these regions need to be treated in some special manner outside of not touching them for IOMMU?