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To:     Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>
Cc:     x86@kernel.org, David Woodhouse <dwmw@infradead.org>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Brian Gerst <brgerst@gmail.com>,
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        Piotr Gorski <lucjan.lucjanov@gmail.com>,
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References: <20230414225551.858160935@linutronix.de>
 <20230414232310.194293270@linutronix.de>
From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
Subject: Re: [patch 16/37] x86/xen/smp_pv: Remove wait for CPU online
In-Reply-To: <20230414232310.194293270@linutronix.de>
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On 4/14/23 7:44 PM, Thomas Gleixner wrote:
> Now that the core code drops sparse_irq_lock after the idle thread
> synchronized, it's pointless to wait for the AP to mark itself online.
> 
> Whether the control CPU runs in a wait loop or sleeps in the core code
> waiting for the online operation to complete makes no difference.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Juergen Gross <jgross@suse.com>
> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> Cc: xen-devel@lists.xenproject.org
> ---
>   arch/x86/xen/smp_pv.c |   10 +++++-----
>   1 file changed, 5 insertions(+), 5 deletions(-)
> 
> --- a/arch/x86/xen/smp_pv.c
> +++ b/arch/x86/xen/smp_pv.c
> @@ -340,11 +340,11 @@ static int xen_pv_cpu_up(unsigned int cp
>   
>   	xen_pmu_init(cpu);
>   
> -	rc = HYPERVISOR_vcpu_op(VCPUOP_up, xen_vcpu_nr(cpu), NULL);
> -	BUG_ON(rc);
> -
> -	while (cpu_report_state(cpu) != CPU_ONLINE)
> -		HYPERVISOR_sched_op(SCHEDOP_yield, NULL);
> +	/*
> +	 * Why is this a BUG? If the hypercall fails then everything can be
> +	 * rolled back, no?
> +	 */


In many cases this indicates either some sort of hypervisor internal error or broken logic in the guest, so it is, well, a bug. But I suppose it may also be some transient condition in the hypervisor (I don't see it now but it can happen in the future) so perhaps we should indeed try not to die on the spot.


-boris


> +	BUG_ON(HYPERVISOR_vcpu_op(VCPUOP_up, xen_vcpu_nr(cpu), NULL));
>   
>   	return 0;
>   }
>