On Wed, Nov 11, 2020 at 09:07:30PM +0100, Peter Zijlstra wrote:
> 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.
But what if you want to live migrate to another hypervisor ;-)
> > 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?
Right, it makes objtool's job a _little_ easier, since it already knows
how to read alternatives. But it still has to learn to deal with the
conflicting stack layouts.
--
Josh
