Hi Michael,
Thanks for mentioning Bochs, I've copied your 486 config commandline and
was able to build and start testing. Its way more handy than with real
iron. It is even possible to disable e820 function by hand.
Now as long as 586+ requirement is not really critical for operation,
maybe put such fragments into conditional ifdefs and introduce some
config option (say LEGACY486) to allow 486-compatible builds? 486s are
still not uncommon in production and network boot is quite usefull in
such environments.
Still no result with my 8139 problem yet, but at least I now have a
convenient testing environment.
Thank you,
Regards,
Nikolai
05.05.2021 15:18, Michael Brown:
A quick test indicates that unlzma.S will still build with ".arch i486".
AFAICT the only non-i386 instruction in there is "bswap", which should
exist on a 486.
I did some quick testing in bochs configured for a 486 using:
./configure --enable-a20-pin --disable-x86_64 --enable-cpu-level=4 \
--enable-pci --enable-e1000 --enable-debugger \
--disable-debugger-gui --enable-readline \
--enable-all-optimizations --enable-cdrom --disable-smp
This showed that iPXE is getting stuck on the "rdtsc" instruction, which
is an undefined opcode on i486.
The rdtsc-based timer isn't causing the problem: the code in
rdtsc_probe() already checks for an invariant TSC (which isn't present
on i486) and so never issues the rdtsc instruction.
There are two remaining uses: in undinet_call() where the TSC value gets
used only for profiling, and in rtc_sample() where the TSC value gets
used as part of entropy generation.
Commenting out these instructions (via the attached patch) gave me an
iPXE binary that worked fine on i486.
Michael
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