[ipxe-devel] 486 with a Realtek 8139

[Michael Brown]

On 05/05/2021 07:54, Martin Habets wrote:
> Long ago Etherboot used to be i386 code.
> Nowadays I think iPXE only runs on i586 or later,
> e.g. arch/x86/prefix/unlzma.S has:
> 	.arch i586
> 
>  From memory there are a couple of places in the code where
> iPXE uses assembler opcodes that don't exist in 486.
> Your best bet is probably using the old Etherboot code base and
> debug/patch that.

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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