On Wed, Nov 11, 2020 at 08:42:06PM +0100, Peter Zijlstra wrote:
> > Would objtool have an easier time coping if this were implemented in
> > terms of a static call?
>
> I doubt it, the big problem is that there is no visibility into the
> actual alternative text. Runtime patching fragments into static call
> would have the exact same problem.
>
> Something that _might_ maybe work is trying to morph the immediate
> fragments into an alternative. That is, instead of this:
>
> static inline notrace unsigned long arch_local_save_flags(void)
> {
> return PVOP_CALLEE0(unsigned long, irq.save_fl);
> }
>
> Write it something like:
>
> static inline notrace unsigned long arch_local_save_flags(void)
> {
> PVOP_CALL_ARGS;
> PVOP_TEST_NULL(irq.save_fl);
> asm_inline volatile(ALTERNATIVE(paravirt_alt(PARAVIRT_CALL),
> "PUSHF; POP _ASM_AX",
> X86_FEATURE_NATIVE)
> : CLBR_RET_REG, ASM_CALL_CONSTRAINT
> : paravirt_type(irq.save_fl.func),
> paravirt_clobber(PVOP_CALLEE_CLOBBERS)
> : "memory", "cc");
> return __eax;
> }
>
> And then we have to teach objtool how to deal with conflicting
> alternatives...
>
> That would remove most (all, if we can figure out a form that deals with
> the spinlock fragments) of paravirt_patch.c
>
> Hmm?
I was going to suggest something similar. Though I would try to take it
further and replace paravirt_patch_default() with static calls.
Either way it doesn't make objtool's job much easier. But it would be
nice to consolidate runtime patching mechanisms and get rid of
.parainstructions.
--
Josh
