Re: [patch 13/26] Xen-paravirt_ops: Consistently wrap paravirt opscallsites to make them patchable

From: Zachary Amsden
Date: Mon Mar 19 2007 - 21:23:49 EST


Jeremy Fitzhardinge wrote:
For example, say we wanted to put a general call for sti into entry.S,
where its expected it won't touch any registers. In that case, we'd
have a sequence like:

push %eax
push %ecx
push %edx
call paravirt_cli
pop %edx
pop %ecx
pop %eax


If we parse the relocs, then we'd find the reference to paravirt_cli. If we look at the byte before and see 0xe8, then we can see if its a
call. If we then work out in each direction and see matched push/pops,
then we know what registers can be trashed in the call. This also
allows us to determine the callsite size, and therefore how much space
we need for inlining.

No, that is a very dangerous suggestion. You absolutely *cannot* do this safely without explicitly marking the start EIP of this code. You *must* use metadata to do that. It is never safe to disassemble backwards or "rewind" EIP for x86 code.

Zach
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