About Adam Young

Once upon a time I was an Army Officer, but that was long ago. Now I work as a Software Engineer. I climb rocks, play saxophone, and spend way too much time in front of a computer.

Enabling ARM64 CPU Capabilities in the Linux Kernel

ARM64 design defines features long before there is a CPU that can implement those features. Since the ARM ecosystem is so varied, there are many different CPU designs out there with different capabilities. A general purpose linux Kernel build put out by a major distribution has to work across a wide array of chips by a large nuymber of vendors. Thus, there is an enumeration of the capabilities inside the Kernel and mechnism for describing how to probe each of these capabilities.

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Finding Linux Kernel Config options in menuconfig

We have reason to believe that we should not be setting CONFIG_EFI_DISABLE_RUNTIME=y In our Kernel configs. I want to perform a controlled expereient booting two Kernel builds, one with this option set and one with it disabled. Since I have the option set, building that Kernel is trivial.

 make olddefconfig
 make -j$(nproc)  rpm-pkg

Now, to turn that option off, I could just edit the .config file. However, it is possible that there are other config options linked to that one, and there is logic to modify them together. I want to see what happens if I use make menuconfig to change the option to confirm (or deny) that only that option gets changed. But where do I find this option in the menu?

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Building and Running the Linux Kernel Selftests on AARCH64/ Fedora

I won’t go into checking out or building the Kernel, as that is covered elsewhere. Assuming you have a buildable Kernel, you can build the tests with:

make -C tools/testing/selftests

But you are probably going to see errors like this:

ksm_tests.c:7:10: fatal error: numa.h: No such file or directory
    7 | #include <numa.h>
      |          ^~~~~~~~
compilation terminated.

The userland test suites use several libraries and need headers to compile the tests that call those libraries. Here is the yum, line I ran to get the dependencies I needed for my system:

sudo yum install libmnl-devel fuse-devel numactl-devel libcap-ng-devel alsa-lib-devel

With those installed, the make line succeeded.

Running the test like this CRASHED THE SYSTEM. Don’t do this.

 make -C tools/testing/selftests run_tests

A more sensible test to run is the example on the Docs page:

# make -C tools/testing/selftests TARGETS=ptrace run_tests
make: Entering directory '/root/linux/tools/testing/selftests'
make --no-builtin-rules ARCH=arm64 -C ../../.. headers_install
make[1]: Entering directory '/root/linux'
  INSTALL ./usr/include
make[1]: Leaving directory '/root/linux'
make[1]: Entering directory '/root/linux/tools/testing/selftests/ptrace'
make[1]: Nothing to be done for 'all'.
make[1]: Leaving directory '/root/linux/tools/testing/selftests/ptrace'
make[1]: Entering directory '/root/linux/tools/testing/selftests/ptrace'
TAP version 13
# selftests: ptrace: get_syscall_info
# TAP version 13
# 1..1
# # Starting 1 tests from 1 test cases.
# #  RUN           global.get_syscall_info ...
# #            OK  global.get_syscall_info
# ok 1 global.get_syscall_info
# # PASSED: 1 / 1 tests passed.
# # Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0
ok 1 selftests: ptrace: get_syscall_info
# selftests: ptrace: peeksiginfo
ok 2 selftests: ptrace: peeksiginfo
# selftests: ptrace: vmaccess
# TAP version 13
# 1..2
# # Starting 2 tests from 1 test cases.
# #  RUN           global.vmaccess ...
# #            OK  global.vmaccess
# ok 1 global.vmaccess
# #  RUN           global.attach ...
# # attach: Test terminated by timeout
# #          FAIL  global.attach
# not ok 2 global.attach
# # FAILED: 1 / 2 tests passed.
# # Totals: pass:1 fail:1 xfail:0 xpass:0 skip:0 error:0
not ok 3 selftests: ptrace: vmaccess # exit=1
make[1]: Leaving directory '/root/linux/tools/testing/selftests/ptrace'
make: Leaving directory '/root/linux/tools/testing/selftests'

Next up is to write my own stub test.

Splitting the irealpro 1400

The irealpro player has become an indispensable tool for practicing a tune. Since I play saxophone, the fact that irealpro will give me a full rhythm section to play against makes working on a tune far more possible than it was for me in the past.

I recently got a new phone and went to reinstall the set of songs that I use. They come from a collection called the Jazz 1400. This snuck up slightly in number from the last time I imported, and it must have hit a threshold, because my phone refuses to import it.

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A Non-authoritative history of Preemptive Multitasking in the personal computing world.

Back when machines only had one or two CPUs (still the case for embedded devices) the OS Kernel was responsible for making sure that the machine coule process more than one instruction “path” at a time. I started coding back on the Commodore 64, and there it was easy to lock up the machine: just run a program that does nothing. I’d have to look back at the Old Programmer’s Guide, but I am pretty sure that a program had to voluntarily give up the CPU if you wanted any form of multi-tasking.

The alternative is called “preemptive multitasking” where the hardware provides a mechanism that can call a controller function to switch tasks. The task running on the CPU is paused, the state is saved, and the controller function decides what to do next.

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