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Date:   Mon, 31 Jul 2023 16:36:31 -0500
From:   Josh Poimboeuf <jpoimboe@kernel.org>
To:     Valentin Schneider <vschneid@redhat.com>
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Subject: Re: [RFC PATCH v2 11/20] objtool: Flesh out warning related to
 pv_ops[] calls
Message-ID: <20230731213631.pywytiwdqgtgx4ps@treble>
References: <20230720163056.2564824-1-vschneid@redhat.com>
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 <20230728153334.myvh5sxppvjzd3oz@treble>
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On Mon, Jul 31, 2023 at 12:16:59PM +0100, Valentin Schneider wrote:
> You're quite right - fabricating an artificial warning with a call to __flush_tlb_local():
> 
>   vmlinux.o: warning: objtool: pv_ops[1]: indirect call to native_flush_tlb_local() leaves .noinstr.text section
>   vmlinux.o: warning: objtool: __flush_tlb_all_noinstr+0x4: call to {dynamic}() leaves .noinstr.text section
> 
> Interestingly the second one doesn't seem to have triggered the "pv_ops"
> bit of call_dest_name. Seems like any call to insn_reloc(NULL, x) will
> return NULL.

Yeah, that's weird.

> Trickling down the file yields:
> 
>   vmlinux.o: warning: objtool: pv_ops[1]: indirect call to native_flush_tlb_local() leaves .noinstr.text section
>   vmlinux.o: warning: objtool: __flush_tlb_all_noinstr+0x4: call to pv_ops[0]() leaves .noinstr.text section
> 
> In my case (!PARAVIRT_XXL) pv_ops should look like:
>   [0]: .cpu.io_delay
>   [1]: .mmu.flush_tlb_user()
> 
> so pv_ops[1] looks right. Seems like pv_call_dest() gets it right because
> it uses arch_dest_reloc_offset().
> 
> If I use the above to fix up validate_call(), would we still need
> pv_call_dest() & co?

The functionality in pv_call_dest() is still needed because it goes
through all the possible targets for the .mmu.flush_tlb_user() pointer
-- xen_flush_tlb() and native_flush_tlb_local() -- and makes sure
they're noinstr.

Ideally it would only print a single warning for this case, something
like:

  vmlinux.o: warning: objtool: __flush_tlb_all_noinstr+0x4: indirect call to native_flush_tlb_local() leaves .noinstr.text section

I left out "pv_ops[1]" because it's already long enough :-)

It would need a little bit of code shuffling.  But it's really a
preexisting problem so don't feel compelled to fix it with this patch
set.

-- 
Josh