linux-kernel - Re: [PATCH 0/8] KVM: minor APIC fixes and cleanups

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Date:	Fri, 30 Jan 2015 16:20:31 +0100
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Nadav Amit <namit@...technion.ac.il>,
	Gleb Natapov <gleb@...nel.org>
Subject: Re: [PATCH 0/8] KVM: minor APIC fixes and cleanups

2015-01-30 10:22+0100, Paolo Bonzini:
> On 29/01/2015 22:48, Radim Krčmář wrote:
> > This patch series is made our of three logical parts,
> > 
> > [1-3/8] are just a cleanup and could be omitted
> > [4-6/8] improve broadcast detection and unoptimized delivery
> > [7-8/8] handle mixed mode (by falling back to improvements from [5-6/9])
> 
> I applied patches 1-4 and commented on patch 5.
> 
> I'm afraid that patches 7-8 could lead to missed optimizations in some cases.

True, I'll do more research on complexity of its assembly.

> Regarding patch 6, perhaps there's a way to avoid this:
> 
> +	/* XXX: A superset of x2APIC broadcasts is fine in practice as long as
> +	 * we don't support APIC ID > 0xfeffffff.
> +	 */
> 
> It's ugly. :)

Yeah, there is: don't deliver x2APIC broadcasts to xAPIC.
(I'm not even sure if it is correct ...)

Without that delivery, we could do something like
  if (irq->dest_id == (x2apic_ipi ? X2APIC_BROADCAST : APIC_BROADCAST))
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