Re: WARNING: can't access registers at asm_common_interrupt

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 11/11/2020 20:15, Josh Poimboeuf wrote:
> 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 ;-)

The same as what happens currently.  The user gets to keep all the
resulting pieces ;)

>>> 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.

I suppose the needed abstraction is "these blocks will start and end
with the same stack layout", while allowing the internals to diverge.

~Andrew