linux-kernel - Re: [PATCH] x86/xen: remove unneeded preempt_disable() from xen_irq

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Message-ID: <99e6cf1d-ce6a-83e5-2e43-12f6c1a89f3f@suse.com>
Date:   Tue, 21 Sep 2021 09:53:38 +0200
From:   Jan Beulich <jbeulich@...e.com>
To:     Juergen Gross <jgross@...e.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, xen-devel@...ts.xenproject.org,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/xen: remove unneeded preempt_disable() from
 xen_irq_enable()

On 21.09.2021 09:02, Juergen Gross wrote:
> --- a/arch/x86/xen/irq.c
> +++ b/arch/x86/xen/irq.c
> @@ -57,24 +57,20 @@ asmlinkage __visible void xen_irq_enable(void)
>  {
>  	struct vcpu_info *vcpu;
>  
> -	/*
> -	 * We may be preempted as soon as vcpu->evtchn_upcall_mask is
> -	 * cleared, so disable preemption to ensure we check for
> -	 * events on the VCPU we are still running on.
> -	 */
> -	preempt_disable();
> -
>  	vcpu = this_cpu_read(xen_vcpu);
>  	vcpu->evtchn_upcall_mask = 0;
>  
> -	/* Doesn't matter if we get preempted here, because any
> -	   pending event will get dealt with anyway. */
> +	/*
> +	 * Now preemption could happen, but this is only possible if an event
> +	 * was handled, so missing an event due to preemption is not
> +	 * possible at all.
> +	 * The worst possible case is to be preempted and then check events
> +	 * pending on the old vcpu, but this is not problematic.
> +	 */

I agree this isn't problematic from a functional perspective, but ...

>  	barrier(); /* unmask then check (avoid races) */
>  	if (unlikely(vcpu->evtchn_upcall_pending))
>  		xen_force_evtchn_callback();

... is a stray call here cheaper than ...

> -
> -	preempt_enable();

... the preempt_{dis,en}able() pair?

Jan