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

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Date:	Wed, 25 Nov 2015 16:43:09 +0100
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	"Wu, Feng" <feng.wu@...el.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 15:38+0100, Paolo Bonzini:
> On 25/11/2015 15:12, Radim Krcmár wrote:
>> 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.)
> 
> Vector hashing is an improvement for the common case where all vectors
> are set to all CPUs.  Sure you can get an unlucky assignment, but it's
> still better than pick-first-APIC.

Yeah, hashing has a valid use case, but a subtle weighting of drawbacks
led me to prefer pick-first-APIC ...

(I'd prefer to have simple code in KVM and depend on static IRQ balancing
 in a guest to handle the distribution.
 The guest could get the unlucky assignment anyway, so it should be
 prepared;  and hashing just made KVM worse in that case.  Guests might
 also configure physical x(2)APIC, where is no lowest priority.
 And if the guest doesn't do anything with IRQs, then it might not even
 care about the impact that our choice has.)
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