Re: [PATCH 1/7] KVM: api: add kvm_irq_routing_extended_msi

[Andre Przywara]

Hi Pavel,

On 02/07/15 08:26, Pavel Fedin wrote:
>  Hello!
> 
>> -----Original Message-----
>> From: kvm-owner@xxxxxxxxxxxxxxx [mailto:kvm-owner@xxxxxxxxxxxxxxx] On Behalf Of Eric Auger
>> Sent: Monday, June 29, 2015 6:37 PM
>> To: eric.auger@xxxxxx; eric.auger@xxxxxxxxxx; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx;
>> marc.zyngier@xxxxxxx; christoffer.dall@xxxxxxxxxx; andre.przywara@xxxxxxx;
>> kvmarm@xxxxxxxxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx
>> Cc: linux-kernel@xxxxxxxxxxxxxxx; patches@xxxxxxxxxx; p.fedin@xxxxxxxxxxx; pbonzini@xxxxxxxxxx
>> 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@xxxxxxxxxx>
>>
>> ---
>>
>> 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.
> 
>> @@ -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.

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. Introducing a new KVM_CAP_... value seems overkill if we could
just have a new routing entry type. So we could still reuse the existing
struct kvm_irq_routing_msi (and extend that with the devid field), but
we would have to add a new routing type number.
Maybe we could collapse this into the existing MSI type + flag when
handing it further down the kernel?

Cheers,
Andre.

> 
>>
>> 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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