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

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Date:   Wed, 22 Sep 2021 12:46:29 +0200
From:   Juergen Gross <jgross@...e.com>
To:     xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org
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>
Subject: Re: [PATCH] x86/xen: remove unneeded preempt_disable() from
 xen_irq_enable()

On 21.09.21 09:02, Juergen Gross wrote:
> Disabling preemption in xen_irq_enable() is not needed. There is no
> risk of missing events due to preemption, as preemption can happen
> only in case an event is being received, which is just the opposite
> of missing an event.
> 
> Signed-off-by: Juergen Gross <jgross@...e.com>

Please ignore this patch, it is superseded now by

"[PATCH v2 0/2] x86/xen: simplify irq pvops"


Juergen

> ---
>   arch/x86/xen/irq.c | 18 +++++++-----------
>   1 file changed, 7 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/xen/irq.c b/arch/x86/xen/irq.c
> index dfa091d79c2e..ba9b14a97109 100644
> --- 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.
> +	 */
>   
>   	barrier(); /* unmask then check (avoid races) */
>   	if (unlikely(vcpu->evtchn_upcall_pending))
>   		xen_force_evtchn_callback();
> -
> -	preempt_enable();
>   }
>   PV_CALLEE_SAVE_REGS_THUNK(xen_irq_enable);
>   
> 


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