From: Miroslav Benes <mbenes@suse.cz>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: "Jürgen Groß" <jgross@suse.com>,
boris.ostrovsky@oracle.com, sstabellini@kernel.org,
tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
hpa@zytor.com, x86@kernel.org, xen-devel@lists.xenproject.org,
linux-kernel@vger.kernel.org, live-patching@vger.kernel.org,
jslaby@suse.cz, "Peter Zijlstra" <peterz@infradead.org>
Subject: Re: [RFC PATCH 2/2] x86/xen: Make the secondary CPU idle tasks reliable
Date: Tue, 17 Mar 2020 10:16:18 +0100 (CET) [thread overview]
Message-ID: <alpine.LSU.2.21.2003171014470.21109@pobox.suse.cz> (raw)
In-Reply-To: <20200316203514.qm7so7b55jbmskgg@treble>
On Mon, 16 Mar 2020, Josh Poimboeuf wrote:
> On Mon, Mar 16, 2020 at 04:51:12PM +0100, Miroslav Benes wrote:
> > > diff --git a/arch/x86/xen/smp_pv.c b/arch/x86/xen/smp_pv.c
> > > index 6b88cdcbef8f..39afd88309cb 100644
> > > --- a/arch/x86/xen/smp_pv.c
> > > +++ b/arch/x86/xen/smp_pv.c
> > > @@ -92,6 +92,7 @@ asmlinkage __visible void cpu_bringup_and_idle(void)
> > > {
> > > cpu_bringup();
> > > boot_init_stack_canary();
> > > + asm volatile (UNWIND_HINT(ORC_REG_UNDEFINED, 0, ORC_TYPE_CALL, 1));
> > > cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
> > > }
> > >
> > > and that seems to work. I need to properly verify and test, but the
> > > explanation is that as opposed to the above, cpu_startup_entry() is on the
> > > idle task's stack and the hint is then taken into account. The unwound
> > > stack seems to be complete, so it could indeed be the fix.
> >
> > Not the correct one though. Objtool rightfully complains with
> >
> > arch/x86/xen/smp_pv.o: warning: objtool: cpu_bringup_and_idle()+0x6a: undefined stack state
> >
> > and all the other hacks I tried ended up in the same dead alley. It seems
> > to me the correct fix is that all orc entries for cpu_bringup_and_idle()
> > should have "end" property set to 1, since it is the first function on the
> > stack. I don't know how to achieve that without the assembly hack in the
> > patch I sent. If I am not missing something, of course.
> >
> > Josh, any idea?
>
> Yeah, I think mucking with the unwind hints in C code is going to be
> precarious. You could maybe have something like
>
> asm("
> UNWIND_HINT_EMPTY\n
> mov $CPUHP_AP_ONLINE_IDLE, %rdi\n
> call cpu_startup_entry\n
> )"
> unreachable();
>
> but that's pretty ugly (and it might not work anyway).
>
> I suppose we could add a new facility to mark an entire C function as an
> "end" point.
I think it would be an overkill for what I perceive as one-off scenario.
Maybe if there are more use cases in the future, but I doubt it.
> But I think it would be cleanest to just do something like your patch
> and have the entry code be asm which then calls cpu_bringup_and_idle().
> That would make it consistent with all other entry code, which all lives
> in asm.
Ack.
Thanks
Miroslav
prev parent reply other threads:[~2020-03-17 9:16 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-12 14:20 [PATCH 0/2] x86/xen: Make idle tasks reliable Miroslav Benes
2020-03-12 14:20 ` [PATCH 1/2] x86/xen: Make the boot CPU idle task reliable Miroslav Benes
2020-03-12 15:04 ` [Xen-devel] " Andrew Cooper
2020-03-12 15:17 ` Miroslav Benes
2020-03-16 14:33 ` Boris Ostrovsky
2020-03-17 9:13 ` Miroslav Benes
2020-03-12 14:20 ` [RFC PATCH 2/2] x86/xen: Make the secondary CPU idle tasks reliable Miroslav Benes
2020-03-13 8:26 ` Jürgen Groß
2020-03-13 9:54 ` Miroslav Benes
2020-03-16 15:51 ` Miroslav Benes
2020-03-16 20:35 ` Josh Poimboeuf
2020-03-17 9:16 ` Miroslav Benes [this message]
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