linux-kernel - Re: [PATCH] KVM: x86: Add lowest-priority support for vt-d posted-interrupts

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Message-ID: <20151125141238.GC13925@potion.brq.redhat.com>
Date:	Wed, 25 Nov 2015 15:12:39 +0100
From:	Radim Krcmár <rkrcmar@...hat.com>
To:	"Wu, Feng" <feng.wu@...el.com>
Cc:	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86: Add lowest-priority support for vt-d
 posted-interrupts

2015-11-25 03:21+0000, Wu, Feng:
> From: Radim Krčmář [mailto:rkrcmar@...hat.com]
>> The hash function just interprets a subset of vector's bits as a number
>> and uses that as a starting offset in a search for an enabled APIC
>> within the destination set?
>> 
>> For example:
>> The x2APIC destination is 0x00000055 (= first four even APICs in cluster
>> 0), the vector is 0b11100000, and bits 10:8 of IntControl are 000.
>> 
>> 000 means that bits 7:4 of vector are selected, thus the vector hash is
>> 0b1110 = 14, so the round-robin effectively does 14 % 4 (because we only
>> have 4 destinations) and delivers to the 3rd possible APIC (= ID 6)?
> 
> In my current implementation, I don't select a subset of vector's bits as
> the number, instead, I use the whole vector number. For software emulation
> p. o. v, do we really need to select a subset of the vector's bits as the base
> number? What is your opinion? Thanks a lot!

I think it's ok to pick any algorithm we like.  It's unlikely that
software would recognize and take advantage of the hardware algorithm
without adding a special treatment for KVM.
(I'd vote for the simple pick-first-APIC lowest priority algorithm ...
 I don't see much point in complicating lowest priority when it doesn't
 deliver to lowest priority CPU anyway.)

I mainly wanted to know what real hardware really does, because there is
a lot of alternatives that still fit into the Xeon documentation.
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