linux-kernel - Re: [v3 23/26] KVM: Update Posted-Interrupts Descriptor when vCPU is preempted

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Date:	Wed, 17 Dec 2014 18:11:00 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Wu, Feng" <feng.wu@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	"x86@...nel.org" <x86@...nel.org>, Gleb Natapov <gleb@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	"dwmw2@...radead.org" <dwmw2@...radead.org>,
	"joro@...tes.org" <joro@...tes.org>,
	Alex Williamson <alex.williamson@...hat.com>,
	jiang.liu-VuQAYsv1563Yd54FQh9/CA@...lic.gmane.org
CC:	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>,
	Eric Auger <eric.auger@...aro.org>
Subject: Re: [v3 23/26] KVM: Update Posted-Interrupts Descriptor when vCPU
 is preempted



On 12/12/2014 16:14, Feng Wu wrote:
> +	if (irq_remapping_cap(IRQ_POSTING_CAP)) {
> +		struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);
> +		struct pi_desc old, new;
> +		unsigned int dest;
> +
> +		memset(&old, 0, sizeof(old));
> +		memset(&new, 0, sizeof(new));

This is quite expensive.  Just use an u64 for old_control and
new_control, instead of a full struct.

> 
> +			pi_clear_sn(&new);

This can be simply new.sn = 0.  It does not need atomic operations.

Same in patch 24 (if needed at all there---see the reply there).

> 
> +	if (irq_remapping_cap(IRQ_POSTING_CAP)) {
> +		struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);
> +		struct pi_desc old, new;
> +
> +		memset(&old, 0, sizeof(old));
> +		memset(&new, 0, sizeof(new));
> +

Here you do not need old/new at all because...

> +		if (vcpu->preempted) {
> +			do {
> +				old.control = new.control = pi_desc->control;
> +				pi_set_sn(&new);
> +			} while (cmpxchg(&pi_desc->control, old.control,
> +					new.control) != old.control);

this can do pi_set_sn directly on pi_desc, without the cmpxchg.

Paolo
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