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Date:   Wed, 11 Nov 2020 14:15:06 -0600
From:   Josh Poimboeuf <jpoimboe@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Andrew Cooper <andrew.cooper3@citrix.com>,
        Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Nicholas Piggin <npiggin@gmail.com>,
        Damien Le Moal <Damien.LeMoal@wdc.com>, jgross@suse.com,
        x86@kernel.org
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