Re: [PATCH] KVM: x86: Convert INIT and SIPI signals into synchronously handled events

[Paolo Bonzini <pbonzini@redhat.com>]

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Il 13/03/2013 08:39, Jan Kiszka ha scritto:
>> I think this should clear pending SIPIs, unless KVM_APIC_INIT
>> was already set in which case it should be a no-op.  Something
>> like:
>> 
>> e = apic->pending_events; while (!(e & KVM_APIC_INIT)) e =
>> cmpxchg(&apic->pending_events, e, (e | KVM_APIC_INIT) &
>> ~KVM_APIC_SIPI);
>> 
>> If you do this, better make pending_events an atomic_t.
> 
> Quick question: Why atomic_t? It becomes a bit ugly to poke into
> the atomic counter for bitops, and cmpxchg is mapped on an
> interlocked version.

It's a bit safer because it forces you to use atomic_read, and
generally points out to reviewers to think about memory barriers.  But
indeed the lack of atomic_set_bit and atomic_clear_bit makes the code
uglier.  And as long as there are no other bits in
apic->pending_events, the above loop is actually the same as just

  apic->pending_events = 1 << KVM_APIC_INIT;

Paolo
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