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

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Date:	Mon, 6 Jul 2015 14:08:37 +0200
From:	Christoffer Dall <christoffer.dall@...aro.org>
To:	Andre Przywara <andre.przywara@....com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	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 Mon, Jul 06, 2015 at 12:23:19PM +0100, Andre Przywara wrote:
> Hi Paolo,
> 
> thanks for looking at this!
> 
> On 06/07/15 12:07, Paolo Bonzini wrote:
> > 
> > 
> > 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?
> 
> Well, as we are about to implement this: yes. But the issue is that MSI
> injection and GSI routing code is generic PCI code in userland (at least
> in kvmtool, guess in QEMU, too), so I don't want to pull in any kind of
> ARM specific code in there. The idea is to always provide the device ID
> from the PCI code (for PCI devices it's just the B/D/F triplet), but
> only send it to the kernel if needed. Querying a KVM capability is
> perfectly fine for this IMO.
> 
> > I
> > guess it's okay to return -EINVAL if the userspace doesn't set the flag
> > but the virtual hardware requires it.
> 
> Yes, that is what I do in the kernel implementation. And that is
> perfectly fine: the ITS emulation does not work without a device ID, the
> ITS driver in the guest assigns the very same payload (and address) to
> different devices, so there is no way to tell the MSIs apart without a
> unique device ID.
> 
Just so I'm sure I understand: The way the kernel differentiates between
no-devid and devid==0, is whether or not the devid flag is set, correct?

-Christoffer
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