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Re: [PATCH] x86/xen: remove unneeded preempt_disable() from xen_irq_enable()
- To: Juergen Gross <jgross@xxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 21 Sep 2021 09:53:38 +0200
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- Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Borislav Petkov <bp@xxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, x86@xxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx
- Delivery-date: Tue, 21 Sep 2021 07:53:53 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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
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