linux-kernel - Re: [patch 16/37] x86/xen/smp_pv: Remove wait for CPU online

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Date:   Mon, 17 Apr 2023 16:46:17 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, David Woodhouse <dwmw@...radead.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Brian Gerst <brgerst@...il.com>,
        Arjan van de Veen <arjan@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Paul McKenney <paulmck@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Sean Christopherson <seanjc@...gle.com>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Paul Menzel <pmenzel@...gen.mpg.de>,
        "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
        Piotr Gorski <lucjan.lucjanov@...il.com>,
        Juergen Gross <jgross@...e.com>,
        xen-devel@...ts.xenproject.org,
        David Woodhouse <dwmw@...zon.co.uk>,
        Usama Arif <usama.arif@...edance.com>,
        Russell King <linux@...linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Guo Ren <guoren@...nel.org>,
        linux-csky@...r.kernel.org,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        linux-mips@...r.kernel.org,
        "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
        Helge Deller <deller@....de>, linux-parisc@...r.kernel.org,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        linux-riscv@...ts.infradead.org,
        Mark Rutland <mark.rutland@....com>,
        Sabin Rapan <sabrapan@...zon.com>
Subject: Re: [patch 16/37] x86/xen/smp_pv: Remove wait for CPU online



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@...utronix.de>
> Cc: Juergen Gross <jgross@...e.com>
> Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> Cc: xen-devel@...ts.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;
>   }
>