Date:	Thu, 2 Jul 2015 16:14:10 +0100
From:	Andre Przywara <andre.przywara@....com>
To:	Eric Auger <eric.auger@...aro.org>,
	Pavel Fedin <p.fedin@...sung.com>,
	"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 Eric,

On 02/07/15 15:49, Eric Auger wrote:
> Hi Pavel,
> On 07/02/2015 09:26 AM, Pavel Fedin wrote:
>>  Hello!
>>
>>> -----Original Message-----
>>> From: kvm-owner@...r.kernel.org [mailto:kvm-owner@...r.kernel.org] On Behalf Of Eric Auger
>>> Sent: Monday, June 29, 2015 6:37 PM
>>> To: eric.auger@...com; eric.auger@...aro.org; linux-arm-kernel@...ts.infradead.org;
>>> marc.zyngier@....com; christoffer.dall@...aro.org; andre.przywara@....com;
>>> kvmarm@...ts.cs.columbia.edu; kvm@...r.kernel.org
>>> Cc: linux-kernel@...r.kernel.org; patches@...aro.org; p.fedin@...sung.com; pbonzini@...hat.com
>>> Subject: [PATCH 1/7] KVM: api: add kvm_irq_routing_extended_msi
>>>
>>> On ARM, the MSI msg (address and data) comes along with
>>> out-of-band device ID information. The device ID encodes the device
>>> that composes the MSI msg. Let's create a new routing entry type,
>>> dubbed KVM_IRQ_ROUTING_EXTENDED_MSI and use the __u32 pad space
>>> to convey the device ID.
>>>
>>> Signed-off-by: Eric Auger <eric.auger@...aro.org>
>>>
>>> ---
>>>
>>> RFC -> PATCH
>>> - remove kvm_irq_routing_extended_msi and use union instead
>>> ---
>>>  Documentation/virtual/kvm/api.txt | 9 ++++++++-
>>>  include/uapi/linux/kvm.h          | 6 +++++-
>>>  2 files changed, 13 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
>>> index d20fd94..6426ae9 100644
>>> --- a/Documentation/virtual/kvm/api.txt
>>> +++ b/Documentation/virtual/kvm/api.txt
>>> @@ -1414,7 +1414,10 @@ struct kvm_irq_routing_entry {
>>>  	__u32 gsi;
>>>  	__u32 type;
>>>  	__u32 flags;
>>> -	__u32 pad;
>>> +	union {
>>> +		__u32 pad;
>>> +		__u32 devid;
>>> +	};
>>>  	union {
>>>  		struct kvm_irq_routing_irqchip irqchip;
>>>  		struct kvm_irq_routing_msi msi;
>>
>>  devid is actually a part of MSI bunch. Shouldn't it be a part of struct kvm_irq_routing_msi then?
>> It also has reserved pad.
> Well this makes sense to me to associate the devid to the msi and put
> devid in the pad field of struct kvm_irq_routing_msi.
> 
> André, Christoffer, would you agree on this change? - I would like to
> avoid doing/undoing things ;-) -

Yes, that makes sense to me. TBH I haven't had a closer look at the
patches yet, but clearly devid belongs into struct kvm_irq_routing_msi.

>>
>>> @@ -1427,6 +1430,10 @@ struct kvm_irq_routing_entry {
>>>  #define KVM_IRQ_ROUTING_IRQCHIP 1
>>>  #define KVM_IRQ_ROUTING_MSI 2
>>>  #define KVM_IRQ_ROUTING_S390_ADAPTER 3
>>> +#define KVM_IRQ_ROUTING_EXTENDED_MSI 4
>>> +
>>> +In case of KVM_IRQ_ROUTING_EXTENDED_MSI routing type, devid is used to convey
>>> +the device ID.
>>>
>>>  No flags are specified so far, the corresponding field must be set to zero.
>>
>> What if we use KVM_MSI_VALID_DEVID flag instead of new KVM_IRQ_ROUTING_EXTENDED_MSI definition? I
>> believe this would make an API more consistent and introduce less new definitions.
> do you mean using type == KVM_IRQ_ROUTING_MSI and flag ==
> KVM_MSI_VALID_DEVID? Not sure this is simpler/clearer. s390 paved the
> way for new routing entry types. I add a new one here.

I tend to agree with Pavel's solution. When hacking IRQ routing support
into kvmtool I saw that it's nasty being forced to differentiate between
the two MSI routing types. Actually userland should be able to query the
kernel about what kind of routing it requires. Also there is the issue
that we must _not_ set the flag on x86, since that breaks older kernels
(due to that check that Eric removes in 3/7).
So from my point of view the cleanest solution would be to always use
KVM_IRQ_ROUTING_MSI, and add the device ID if the kernel needs it (true
for ITS guests, false for GICv2M, x86, ...)
I am looking for a clever solution for this now.

Cheers,
Andre.

> 
> Another solution may be to use new KVM_IRQ_ROUTING_EXTENDED_MSI type and
> add struct kvm_msi ext_msi in kvm_irq_routing_entry union. It is 8 words
> as well. But most probably this is even uglier.

> 
> Let's see if this thread is heading to a consensus...
> 
> Best Regards
> 
> Eric
>>
>>>
>>> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
>>> index 2a23705..8484681 100644
>>> --- a/include/uapi/linux/kvm.h
>>> +++ b/include/uapi/linux/kvm.h
>>> @@ -841,12 +841,16 @@ struct kvm_irq_routing_s390_adapter {
>>>  #define KVM_IRQ_ROUTING_IRQCHIP 1
>>>  #define KVM_IRQ_ROUTING_MSI 2
>>>  #define KVM_IRQ_ROUTING_S390_ADAPTER 3
>>> +#define KVM_IRQ_ROUTING_EXTENDED_MSI 4
>>>
>>>  struct kvm_irq_routing_entry {
>>>  	__u32 gsi;
>>>  	__u32 type;
>>>  	__u32 flags;
>>> -	__u32 pad;
>>> +	union {
>>> +		__u32 pad;
>>> +		__u32 devid;
>>> +	};
>>>  	union {
>>>  		struct kvm_irq_routing_irqchip irqchip;
>>>  		struct kvm_irq_routing_msi msi;
>>> --
>>> 1.9.1
>>>
>>> --
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>>
>> Kind regards,
>> Pavel Fedin
>> Expert Engineer
>> Samsung Electronics Research center Russia
>>
> 
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