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

[Zachary Amsden <zach@xxxxxxxxxx>]

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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