Re: WARNING: can't access registers at asm_common_interrupt

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Nov 11, 2020 at 01:59:00PM -0600, Josh Poimboeuf wrote:
> 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.

Possible, we just need to be _really_ careful to not allow changing
those static_call()s. So maybe we need DEFINE_STATIC_CALL_RO() which
does a __ro_after_init on the whole thing.

> 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.

I think the above (combining alternative and paravirt/static_call) does
make objtool's job easier, since then we at least have the actual
alternative instructions available to inspect, or am I mis-understanding
things?