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Date:   Wed, 9 Sep 2020 10:12:39 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     "Christopherson, Sean J" <sean.j.christopherson@intel.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        "torvalds@linux-foundation.org" <torvalds@linux-foundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        Will Deacon <will@kernel.org>
Subject: Re: [GIT PULL] First batch of KVM changes for Linux 5.9
Message-ID: <20200909081239.GA2446260@gmail.com>
References: <20200805182606.12621-1-pbonzini@redhat.com>
 <20200908180939.GA2378263@gmail.com>
 <6a83e6f1e9c34e44ae818ef88ec185a7@intel.com>
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* Christopherson, Sean J <sean.j.christopherson@intel.com> wrote:

> Ingo Molnar wrote:
> > * Paolo Bonzini <pbonzini@redhat.com> wrote:
> > 
> > > Paolo Bonzini (11):
> > >       Merge branch 'kvm-async-pf-int' into HEAD
> > 
> > kvmtool broke in this merge window, hanging during bootup right after CPU bringup:
> > 
> >  [    1.289404]  #63
> >  [    0.012468] kvm-clock: cpu 63, msr 6ff69fc1, secondary cpu clock
> >  [    0.012468] [Firmware Bug]: CPU63: APIC id mismatch. Firmware: 3f APIC: 14
> >  [    1.302320] kvm-guest: KVM setup async PF for cpu 63
> >  [    1.302320] kvm-guest: stealtime: cpu 63, msr 1379d7600
> > 
> > Eventually trigger an RCU stall warning:
> > 
> >  [   22.302392] rcu: INFO: rcu_sched detected stalls on CPUs/tasks:
> >  [   22.302392] rcu: 	1-...!: (68 GPs behind) idle=00c/0/0x0 softirq=0/0 fqs=0  (false positive?)
> > 
> > I've bisected this down to the above merge commit. The individual commit:
> > 
> >    b1d405751cd5: ("KVM: x86: Switch KVM guest to using interrupts for page ready APF delivery")
> > 
> > appears to be working fine standalone.
> > 
> > I'm using x86-64 defconfig+kvmconfig on SVM. Can send more info on request.
> > 
> > The kvmtool.git commit I've tested is 90b2d3adadf2.
> 
> Looks a lot like the lack of APIC EOI issue that Vitaly reported[*].
> 
> ---
>  arch/x86/kernel/kvm.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index d45f34cbe1ef..9663ba31347c 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -271,6 +271,8 @@ DEFINE_IDTENTRY_SYSVEC(sysvec_kvm_asyncpf_interrupt)
>  	struct pt_regs *old_regs = set_irq_regs(regs);
>  	u32 token;
>  
> +	ack_APIC_irq();
> +
>  	inc_irq_stat(irq_hv_callback_count);
>  
>  	if (__this_cpu_read(apf_reason.enabled)) {
> --
> 
> [*] https://lkml.kernel.org/r/20200908135350.355053-1-vkuznets@redhat.com

Yep, this does the trick, thanks!

Tested-by: Ingo Molnar <mingo@kernel.org>

	Ingo