linux-kernel - Re: [PATCH 3/5] x86/ioapic: Handle Extended Destination ID field in RTE

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Date:   Thu, 8 Oct 2020 11:12:25 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     x86@...nel.org, kvm <kvm@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/5] x86/ioapic: Handle Extended Destination ID field in
 RTE

On Wed, Oct 07, 2020 at 01:20:44PM +0100, David Woodhouse wrote:
> @@ -1861,7 +1863,8 @@ static void ioapic_configure_entry(struct irq_data *irqd)
>  	 * ioapic chip to verify that.
>  	 */
>  	if (irqd->chip == &ioapic_chip) {
> -		mpd->entry.dest = cfg->dest_apicid;
> +		mpd->entry.dest = cfg->dest_apicid & 0xff;
> +		mpd->entry.ext_dest = cfg->dest_apicid >> 8;
>  		mpd->entry.vector = cfg->vector;
>  	}
>  	for_each_irq_pin(entry, mpd->irq_2_pin)

All the other sites did memset(0) before the assignment, and this the
extra unconditional write of 0 to ext_dest is harmless.

This might be true for this site too, but it wasn't immediately obvious.