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

[Andre Przywara <andre.przywara@arm.com>]

Hi,

On 03/07/15 10:05, Andre Przywara wrote:
> Hi Pavel,
> 
> On 02/07/15 08:26, Pavel Fedin wrote:
>>  Hello!
>>
>>> -----Original Message-----
>>> From: kvm-owner@vger.kernel.org [mailto:kvm-owner@vger.kernel.org] On Behalf Of Eric Auger
>>> Sent: Monday, June 29, 2015 6:37 PM
>>> To: eric.auger@st.com; eric.auger@linaro.org; linux-arm-kernel@lists.infradead.org;
>>> marc.zyngier@arm.com; christoffer.dall@linaro.org; andre.przywara@arm.com;
>>> kvmarm@lists.cs.columbia.edu; kvm@vger.kernel.org
>>> Cc: linux-kernel@vger.kernel.org; patches@linaro.org; p.fedin@samsung.com; pbonzini@redhat.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@linaro.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.
>>
>>> @@ -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?

FWIW, I gave this a try, this doesn't look to bad. I carried the new
type down till virt/kvm/arm/vgic.c:kvm_set_routing_entry(), where the
EXTENDED type got turned back into the normal MSI type while setting the
flag in the internal struct kvm_kernel_irq_routing_entry. This keeps the
new type only to the userland facing side, with the kernel code staying
mostly the same.
Together with a new KVM_CAP_MSIS_REQUIRE_DEVID capability I can now
drive both GICv2M and ITS emulation from the same userland base in a
sane manner.
If someone wants to have a look now, tell me, otherwise I will wait for
Eric's upcoming code drop and comment on that then.

Cheers,
Andre.

> 
> 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
>>>
>>> --
>>> To unsubscribe from this list: send the line "unsubscribe kvm" in
>>> the body of a message to majordomo@vger.kernel.org
>>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>>
>> Kind regards,
>> Pavel Fedin
>> Expert Engineer
>> Samsung Electronics Research center Russia
>>