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

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Date:	Mon, 06 Jul 2015 17:01:40 +0200
From:	Eric Auger <eric.auger@...aro.org>
To:	Pavel Fedin <p.fedin@...sung.com>,
	'Paolo Bonzini' <pbonzini@...hat.com>,
	'Andre Przywara' <andre.przywara@....com>,
	'Christoffer Dall' <christoffer.dall@...aro.org>
CC:	eric.auger@...com, linux-arm-kernel@...ts.infradead.org,
	'Marc Zyngier' <Marc.Zyngier@....com>,
	kvmarm@...ts.cs.columbia.edu, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/7] KVM: api: add kvm_irq_routing_extended_msi

Hi all,
On 07/06/2015 03:32 PM, Pavel Fedin wrote:
>  Hi!
> 
>>> 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.
>>
>> Yes, I agree.
> 
>  Actually, we already have this capability, it's KVM_CAP_IRQ_ROUTING. If we have this capability,
> and want to use irqfds with GICv3, we need to set KVM_MSI_VALID_DEVID. And there is no other way to
> use irqfds with GICv3.
>  Just for example, this is what i have done in qemu:
> --- cut ---
> int kvm_irqchip_add_msi_route(KVMState *s, MSIMessage msg, PCIDevice *dev)
> {
>     struct kvm_irq_routing_entry kroute = {};
>     int virq;
> 
>     if (kvm_gsi_direct_mapping()) {
>         return kvm_arch_msi_data_to_gsi(msg.data);
>     }
> 
>     if (!kvm_gsi_routing_enabled()) {
>         return -ENOSYS;
>     }
> 
>     virq = kvm_irqchip_get_virq(s);
>     if (virq < 0) {
>         return virq;
>     }
> 
>     kroute.gsi = virq;
>     kroute.type = KVM_IRQ_ROUTING_MSI;
>     kroute.u.msi.address_lo = (uint32_t)msg.address;
>     kroute.u.msi.address_hi = msg.address >> 32;
>     kroute.u.msi.data = le32_to_cpu(msg.data);
>     kroute.flags = kvm_msi_flags;
>     if (kroute.flags & KVM_MSI_VALID_DEVID) {
>         kroute.u.msi.devid = (pci_bus_num(dev->bus) << 8) | dev->devfn;
>     }
> 
>     if (kvm_arch_fixup_msi_route(&kroute, msg.address, msg.data)) {
>         kvm_irqchip_release_virq(s, virq);
>         return -EINVAL;
>     }
> 
>     kvm_add_routing_entry(s, &kroute);
>     kvm_irqchip_commit_routes(s);
> 
>     return virq;
> }
> --- cut ---
> 
>  ITS code in qemu just does:
> 
> ---cut ---
>     msi_supported = true;
>     kvm_msi_flags = KVM_MSI_VALID_DEVID;
>     kvm_msi_via_irqfd_allowed = kvm_has_gsi_routing();
>     kvm_gsi_routing_allowed = kvm_msi_via_irqfd_allowed;
> --- cut ---
> 
>  I set KVM_MSI_VALID_DEVID unconditionally here just because it will never be checked if
> kvm_msi_via_irqfd_allowed is false, it's just qemu specifics. The more canonical form would perhaps
> be:
> --- cut ---
> if (kvm_has_gsi_routing()) {
>     kvm_msi_flags = KVM_MSI_VALID_DEVID;
Personally I prefer a capability rather than hardcoding a global
variable value in the qemu interrupt controller code. All the more so
typically there is KVM GSI routing cap that could/should? be queried
instead of hardcoding the value I think.

So not sure whether we eventually concluded;-)
- introduce a KVM_CAP_MSI_DEVID capability? All OK except Pavel not
convinced?
- userspaces puts the devid in struct kvm_irq_routing_msi pad field:
consensus (we do not intrduce a new kvm_irq_routing_ext_msi)
- userspace tells it conveyed a devid by setting
A) the kvm_irq_routing_entry's field?
B) the kvm_irq_routing_entry's type
no consensus. If there is a cap, does it really matter?

Best Regards

Eric
>     kvm_gsi_routing_allowed = true;
>     kvm_msi_via_irqfd_allowed = true;
> }
> --- cut ---
> 
>  I can post my sets as RFCs to qemu mailing list, if you want to take a look at the whole change
> set.
> 
> Kind regards,
> Pavel Fedin
> Expert Engineer
> Samsung Electronics Research center Russia
> 
> 

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