Re: [PATCH] x86/xen: remove unneeded preempt_disable() from xen_irq_enable()

[Juergen Gross <jgross@xxxxxxxx>]

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@xxxxxxxx>
Please ignore this patch, it is superseded now by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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