Re: [PATCH] WTF is VLI?

[Keith Owens <kaos@ocs.com.au>]

On Tue, 16 Nov 2004 09:56:34 +0000 (GMT), 
Hugh Dickins <hugh@veritas.com> wrote:
>On Tue, 16 Nov 2004, Keith Owens wrote:
>> 
>> ksymoops has to work with lots of different log formats from lots of
>> different architectures.  Some arch's already print the code around the
>> oops and enclose the failing instruction in <> or [], some do not.
>> 
>> Just looking at a code string, you cannot tell if the arch has variable
>> length instructions or not (don't forget that ksymoops also works cross
>> architecture).  The VLI tag will work for _all_ architectures that have
>> variable length instructions, not just i386.  At the very least, s390
>> can use it as well.
>
>But, to an outsider, it seems that the "VLI" can only be relevant when
>disassembling the "Code:", and surely each arch disassembler knows
>already if it's dealing with Variable Length Instructions.

ksymoops takes the Code: line, converts it to a suitable object file,
passes that object to the arch specific disassembler then ksymoops
reformats the output to match the kernel symbol table.  With fixed
length instructions, ksymoops can dump the entire code line into a
single object.  With variable length instructions, disassembling the
code before the instruction pointer is dodgy, it may or may not work.

So for VLI code, ksymoops splits the code line into two separate pieces
and processes each one seperately.  ksymoops prints the first bit with
a warning that it may not be reliable.  The second bit, and all the
code line for non-VLI architectures, is reliable and is printed without
a warning.

The VLI tag, together with <> or [] around the failing instruction,
tells ksymoops if this oops needs to be processed in one chunk or two.
_Before_ passing the synthesized object(s) to the disassembler.