openhcl: report bootloader log via device tree when configured

[chris-oo]

At times, it can be annoying to configure com3 or the right logging channel. Add a mechanism for the bootshim to report logs to usermode via device tree.

Today, this is viewable via inspect, but the output isn't the prettiest:

openvmm> x -v vm/runtime_params/parsed_openhcl_boot/
{
    accepted_ranges: _,
    boot_log: "openhcl_boot: early debugging enabled\nreclaim device tree memory 1ef5b814000-1ef5b854000\nopenhcl_boot: entered shim_main\n",
...
}

Obviously this doesn't work when we don't actually make it to usermode.

#710

[daprilik]

talked about this offline a bit - my gut feeling is that stuffing these logs into the DT directly as a string node seems highly unorthadox. A far more natural representation, albeit requiring a bit more work, would be to report a pointer to the page(s) that contain the bootloader logs, which we can then read

[smalis-msft]

It feels a little weird to me too to be stuffing information that isn't really related to devices/configuration into device tree like this. Is there nowhere else we could stash it?

[jstarks]

I think Daniel's right. It's reasonable to report the memory range for the log via devicetree--that's what it's for, to report the VM's topology.

I don't think it's terrible to put the strings in here directly, but it is unorthodox, and it could create pressure on the max size of the dtb if the log gets big.

[chris-oo]

I think i'll revisit this with a different approach - have dtb describe a log buffer region that the bootloader has used.