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Date:   Mon, 31 Jul 2023 23:46:12 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Josh Poimboeuf <jpoimboe@kernel.org>
Cc:     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: <20230731214612.GC51835@hirez.programming.kicks-ass.net>
References: <20230720163056.2564824-1-vschneid@redhat.com>
 <20230720163056.2564824-12-vschneid@redhat.com>
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 <20230731213631.pywytiwdqgtgx4ps@treble>
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On Mon, Jul 31, 2023 at 04:36:31PM -0500, Josh Poimboeuf wrote:
> 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

But then what for the case where there are multiple implementations and
more than one isn't noinstr? IIRC that is where these double prints came
from. One is the callsite (always one) and the second is the offending
implementation (but there could be more).

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

The index number is useful when also looking at the assembler, which
IIRC is an indexed indirect call.