Re: [RFC, PATCH 2/5] Paravirt_ops patch bugs.patch

[Zachary Amsden <zach@vmware.com>]

Jeremy Fitzhardinge wrote:
> Zachary Amsden wrote:
>   
>> Failing to patch because not enough space is available for a call or jump
>> or because the site clobbers do not allow the target clobbers to fit is
>> a fatal error; it means the kernel can not be properly virtualized.
>>   
>>     
>
> No, that doesn't follow.  If the original site was:
>
> patchable_start:
>     push %eax
>     push %ecx
>     push %edx
>     call *paravirt_ops + thingy
>     pop
>     pop
>     pop
> patchable_end:
>
> then its perfectly OK to leave it as-is, even if the direct call's
> destination clobbers are mismatched.  If the patcher wants to generate a
> call to a C function in a context which can't deal with normal C calling
> conventions, then it needs to also patch in appropriate save/restores.
>   

The example is a bit misconstrued.  In this case, the clobbers for the 
patchable region are CLBR_ALL - so there is no possibility of mismatch 
because of expanded clobber list.  If the patchable region consisted of 
this, it would be bad:

    push %eax
    push %ecx
patchable_start:
    push %edx
    call *paravirt_ops + thingy
    pop
patchable_end: (note - site clobber EDX ok)
    pop
    pop


But, why would you do that?

Calls through paravirt_ops function pointers are C function calls.  Failing to provide a patchable region which can make a C function call is a BUG().

Zach