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SubjectRe: [discuss] booting a kernel compiled with -mregparm=0
On Sat, 15 Jan 2005, H. Peter Anvin wrote:
> It depends on the architecture ABI. This is the case for the i386
> ABI, but definitely *NOT* for x86-64.

<begin stuff superseded by later findings, but left here to clarify the
train of thoughts>
Yes, precisely. The ABI/x86_64 defines this behaviour explicitly. However,
that would mean the ABI was designed without giving thought to debugging
via kdb.

When I said "2-3 weeks of work" I didn't fully realize the complexity of
the problem. It is actually more like several months of research work and
then (most likely) coming to the conclusion that the code to simulate the
cpu (by disassembling the functions to track down where those registers
went in each function) is just too complex to be written.

So, this means there is no way for kdb on x86_64 to show the parameter
values for each function in the back trace. Any chance of changing the
ABI/x86_64 to do the right (i.e. passing via stack like on i386) thing
now? Then kdb would automatically support it via normal ar-handling code.

Also, we should NOT claim that Linux has been ported to x86_64
architecture yet, because the port is not clean, i.e. doesn't allow
breaking ABI like it allows on i386, by means of CONFIG_REGPARM
optimization. Anyone working on fixing this?

I cc'd Linus as I cannot believe he agreed with allowing such an
optimization to be a default and standard thing accepted by the Linux
kernel. (But I may be wrong, especially since Linus isn't particularly
fond of kdb anyway :)
<end of stuff declared as superceded>

Actually, having cc'd Linus made me think very _carefully_ about what I
say and I went and checked how the userspace does it, as I couldn't
believe that such fine piece of software as gdb would be broken as well.
And to my surprize I discovered that gdb (when a program is compiled with
-g) works fine! I.e. it shows the function arguments correctly. And
disassembling the functions shows that although the arguments are passed
via registers (as ABI demands) they are also saved somewhere on the stack.
Hmmm, interesting, then -g compiled Linux kernel should also be useable,
with perhaps some tweaks to kdb to decode these frames correctly, right?

I just want someone who knows the answer to confirm the last statement and
so then I can go ahead and (try to) implement the solution by mimicking
gdb behaviour (or help Jack Vogel implement it if he prefers doing it
himself as defacto kdb/x86_64 maintainer)

Kind regards
Tigran
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