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

On 21.09.21 09:53, Jan Beulich wrote:
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))

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

-       preempt_enable();

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

The question is if a stray call in case of preemption (very unlikely)
is cheaper than the preempt_{dis|en}able() pair on each IRQ enabling.

I'm quite sure removing the preempt_*() calls will be a net benefit.


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