linux-kernel - Re: WARNING: can't access registers at asm_common

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Date:   Wed, 11 Nov 2020 13:59:00 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andrew Cooper <andrew.cooper3@...rix.com>,
        Shinichiro Kawasaki <shinichiro.kawasaki@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Damien Le Moal <Damien.LeMoal@....com>, jgross@...e.com,
        x86@...nel.org
Subject: Re: WARNING: can't access registers at asm_common_interrupt

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.

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.

-- 
Josh