Messages in this thread | | | Date | Wed, 11 Nov 2020 14:15:06 -0600 | From | Josh Poimboeuf <> | Subject | Re: WARNING: can't access registers at asm_common_interrupt |
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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
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