[PowerPC] fix panic reading /dev/kmem on !DMAP machines

[alfredo]

Summary

This fixes /dev/kmem causing panic on machines not using DMAP.
The issue was found while running "lib/libkvm/kvm_geterr_test:kvm_geterr_positive_test_no_error" test case on QEMU VM without Huge Pages support.

Test Plan

Run 'kyua test -k /usr/tests/Kyuafile lib/libkvm/kvm_geterr_test:kvm_geterr_positive_test_no_error' on QEMU powerpc64 VM with and without huge pages

[alfredo]

test case improvement moved to D23783

[alfredo]

updated to match behavior of amd64/arm64 when handling an error.

[luporl]

sys/powerpc/powerpc/mem.c

157

Don't you need to break here too (from the for and the while), instead of returning an error?

166

IIRC, you have noticed that kernacc() panics when !hw_direct_map and va was in DMAP range, because there were no vm_map's for DMAP range.
However, read/write access should also be checked when !hw_direct_map.

Instead, we should check if va is outside the DMAP range, or, to match the previous if logic, check if va is inside VM range:
(va >= VM_MIN_KERNEL_ADDRESS) && (va <= virtual_end)

167

A space is needed between ')' and '{'.

[alfredo]

sys/powerpc/powerpc/mem.c

157

I decided to not break here because this is a check made prior any read/write, so it's a case of a "hard" error that I would like to return immediatelly.

We can set `error=EFAULT``` and break, then rely on "if (uio->uio_resid != orig_resid)" to return the error correctly. Do you have any preference?

166

Yeah, good reminder. I'll work on that, and it will require more tests.

[luporl]

sys/powerpc/powerpc/mem.c

157

I decided to not break here because this is a check made prior any read/write

Are you sure about that? This part is inside an outer while.
And, BTW, the break you've added below also occurs before the call to uiomove.

[alfredo]

Adresses reviewers suggestions.

[luporl]

Nice, it looks good to me.