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Date:   Thu, 20 Jan 2022 11:57:14 +0000
From:   Mark Rutland <mark.rutland@arm.com>
To:     Christian Borntraeger <borntraeger@linux.ibm.com>
Cc:     linux-kernel@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        aleksandar.qemu.devel@gmail.com, alexandru.elisei@arm.com,
        anup.patel@wdc.com, aou@eecs.berkeley.edu, atish.patra@wdc.com,
        bp@alien8.de, catalin.marinas@arm.com, chenhuacai@kernel.org,
        dave.hansen@linux.intel.com, frankja@linux.ibm.com,
        frederic@kernel.org, gor@linux.ibm.com, hca@linux.ibm.com,
        james.morse@arm.com, jmattson@google.com, joro@8bytes.org,
        luto@kernel.org, maz@kernel.org, mingo@redhat.com,
        nsaenzju@redhat.com, palmer@dabbelt.com, paulmck@kernel.org,
        paul.walmsley@sifive.com, peterz@infradead.org, seanjc@google.com,
        suzuki.poulose@arm.com, svens@linux.ibm.com, tglx@linutronix.de,
        tsbogend@alpha.franken.de, vkuznets@redhat.com,
        wanpengli@tencent.com, will@kernel.org,
        Anup Patel <anup@brainfault.org>,
        Atish Patra <atishp@atishpatra.org>
Subject: Re: [PATCH v2 0/7] kvm: fix latent guest entry/exit bugs
Message-ID: <YelOGjuRsMfUb3e7@FVFF77S0Q05N>
References: <20220119105854.3160683-1-mark.rutland@arm.com>
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 <20220119192217.GD43919@C02TD0UTHF1T.local>
 <0654e667-1cfa-5147-6661-b3b63288be0b@linux.ibm.com>
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On Wed, Jan 19, 2022 at 08:30:17PM +0100, Christian Borntraeger wrote:
> 
> 
> Am 19.01.22 um 20:22 schrieb Mark Rutland:
> > On Wed, Jan 19, 2022 at 07:25:20PM +0100, Christian Borntraeger wrote:
> > > Am 19.01.22 um 11:58 schrieb Mark Rutland:
> > > 
> > > 
> > > CCing new emails for Anup and Atish so that they are aware of this thread.
> > 
> > Ah; whoops. I'd meant to fix the Ccs on the patches.
> > 
> > Thanks!
> > 
> > [...]
> > 
> > > I just gave this a spin on s390 with debugging on and I got the following:
> > > 
> > > [  457.151295] ------------[ cut here ]------------
> > > [  457.151311] WARNING: CPU: 14 PID: 0 at kernel/rcu/tree.c:613 rcu_eqs_enter.constprop.0+0xf8/0x118
> > 
> > Hmm, so IIUC that's:
> > 
> > 	WARN_ON_ONCE(rdp->dynticks_nmi_nesting != DYNTICK_IRQ_NONIDLE);
> > 
> > ... and we're clearly in the idle thread here.
> > 
> > I wonder, is the s390 guest entry/exit *preemptible* ?
> 
> Looks like debug_defconfig is indeed using preemption:
> 
> CONFIG_PREEMPT_BUILD=y
> # CONFIG_PREEMPT_NONE is not set
> # CONFIG_PREEMPT_VOLUNTARY is not set
> CONFIG_PREEMPT=y
> CONFIG_PREEMPT_COUNT=y
> CONFIG_PREEMPTION=y
> CONFIG_PREEMPT_RCU=y
> CONFIG_PREEMPT_NOTIFIERS=y
> CONFIG_DEBUG_PREEMPT=y
> CONFIG_PREEMPTIRQ_TRACEPOINTS=y
> CONFIG_TRACE_PREEMPT_TOGGLE=y
> CONFIG_PREEMPT_TRACER=y
> # CONFIG_PREEMPTIRQ_DELAY_TEST is not set

Thanks for confirming!

Could you try with CONFIG_PROVE_RCU=y ? That can't be selected directly, but
selecting PROVE_LOCKING=y will enable it.

If I'm right, with that we should get a splat out of
rcu_irq_exit_check_preempt().

If so, I think we can solve this with preempt_{disable,enable}() around the
guest_timing_{enter,exit}_irqoff() calls. We'll also need to add some more
comments around arch_in_rcu_eqs() that arch-specific EQSs should be
non-preemptible.

Thanks,
Mark.