This means QEMU will run slower, but I want to remove any kind of grief I can to simplify my debugging environment. Here is a quick run down of the way I drive this:įirstly, I recommend removing KVM as this has caused me some grief catching breakpoints. QEMU is one very powerful tool - and combined with gdb this has allowed me to debug Intel based boot loaders. Probably an easier way to get debug out is just using the boot option:
Overall, I may have had problems with the host/target and/or the NET20DC-USB host-to-host device, but it did allow me to get some debug out, be it rather unreliably.
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Also, the EHCI USB driver initialisation later on in the kernel initialisation hung which wasn't useful. My experience with this approach wasn't great - I had to plug/unplug the debug device quite frequently for the earlyprintk EHCI reset and probe to work.
So that I won't bore you with the details, this is all explained in the kernel documentation in Documentation/x86/earlyprintk.txtĪs it was, I needed to tweak the earlyprintk driver to put in some delays in the EHCI probing and reset code to get it working on my fairly fast target laptop. On my debug set-up I used a NET20DC-USB Hi-Speed USB 2.0 Host-to-Host Debug Device connecting the target machine and a host with which I capture the USB debug using /dev/ttyUSB0 with minicom.