linux-kernel - Re: [PATCH 1/7] KVM: api: add kvm_irq_routing_extended

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Date:	Mon, 6 Jul 2015 09:30:20 +0100
From:	Andre Przywara <andre.przywara@....com>
To:	Pavel Fedin <p.fedin@...sung.com>,
	"'Eric Auger'" <eric.auger@...aro.org>,
	"eric.auger@...com" <eric.auger@...com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Marc Zyngier <Marc.Zyngier@....com>,
	"christoffer.dall@...aro.org" <christoffer.dall@...aro.org>,
	"kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>
Subject: Re: [PATCH 1/7] KVM: api: add kvm_irq_routing_extended_msi

Hi Pavel,

On 06/07/15 07:42, Pavel Fedin wrote:
>  Hello!
> 
>> I like this approach, but it runs into problems:
>> As you read above the current documentation says that the flags field
>> must be zero and the current KVM_SET_GSI_ROUTING handler bails out if it
>> isn't. So userland would need to know whether it's safe to set that
>> field.
> 
>  This problem does not exist because:
> a) Older platforms do not need this flag, so they expect to get zero.
> b) ARM64 + GICv3 platform cannot work without this flag.
> 
>  This is perfectly OK combination IMO. Userland just knows, whether it needs to supply device ID or
> not. For example, my modified qemu now has kvm_msi_flags global variable which defaults to 0. ITS
> code, then, if activated, changes it to KVM_MSI_VALID_DEVID, and qemu starts supplying device IDs to
> the related calls.

Well, I had this solution before in kvmtool: If ARM && ITS then set the
flag. But I wasn't really happy with this, as the IRQ routing, setup and
injection code is rather architecture agnostic (implementing the generic
KVM interface), so spraying in some architecture hacks sounded not very
elegant.
Also as the flag describes a rather generic feature (provide an unique
device ID), I'd rather avoid to make this an ARM hack.

That being said this is not a show stopper for me, so if the others are
happy with this, I will go down your road.

Cheers,
Andre.
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