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

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Date:	Mon, 6 Jul 2015 11:30:26 +0200
From:	Christoffer Dall <christoffer.dall@...aro.org>
To:	Andre Przywara <andre.przywara@....com>
Cc:	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>,
	"kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"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

On Mon, Jul 06, 2015 at 09:30:20AM +0100, Andre Przywara wrote:
> 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.
> 
There must be some way for userspace to discover if it's valid to set
the flag or not; either through a well-defined error-code probing
mechanism for KVM_SET_GSI_ROUTING or through advertising a capability.

-Christoffer
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