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

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Date:	Mon, 6 Jul 2015 13:07:16 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Christoffer Dall <christoffer.dall@...aro.org>,
	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>
Subject: Re: [PATCH 1/7] KVM: api: add kvm_irq_routing_extended_msi



On 06/07/2015 12:37, Christoffer Dall wrote:
> I don't view it as 'the kernel requires this' but as 'the kernel will
> not complain with arbitrary error code if you set the devid flag'
> capability, and it's up to userspace (as usual) to provide the correct
> arguments for things to work, and up to the kernel to ensure we don't
> crash the system etc.
> 
> Thus, if you want to advertise it as a capability, I would rather call
> it KVM_CAP_MSI_DEVID.

I agree.  Does userspace know that ITS guests always require devid?  I
guess it's okay to return -EINVAL if the userspace doesn't set the flag
but the virtual hardware requires it.

Paolo

> The question is if userspace code that sets the devid flag will anyway
> depend on some discovery mechanism of whether or not the kernel supports
> arm64 irqfd etc. and if so, can we be sure to add the required support
> at once in the kernel so that EINVAL never means 'you set the flags
> field on the ioctl on an old kernel'?
> 
> This smells an awful lot like a capability to me.
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