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
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