The SBI memory was mapped outside the kernel window. This would trigger
an instruction access fault on some hardware, when the bootloader has set
up PMP to protect the SBI memory. Instead of protecting a particular memory
region, the PMP locks the entire page table entry that covers the region.
The PADDR_LOAD and kernel base are adjusted to have the SBI memory
included in the kernel window. So that the SBI memory can be mapped as
part of the kernel image to a separate page table entry. This also
avoids the kernel to allocate untyped memory from the SBI region.
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