Re: [RFC v7 1/7] KVM: api: pass the devid in the msi routing entry

[Auger Eric <eric.auger@redhat.com>]

Hi,

On 21/07/2016 19:15, Radim Krčmář wrote:
> 2016-07-21 17:43+0100, Andre Przywara:
>> Hi Radim,
>>
>> On 21/07/16 17:01, Radim Krčmář wrote:
>>> 2016-07-18 13:25+0000, Eric Auger:
>>>> On ARM, the MSI msg (address and data) comes along with
>>>> out-of-band device ID information. The device ID encodes the
>>>> device that writes the MSI msg. Let's convey the device id in
>>>> kvm_irq_routing_msi and use KVM_MSI_VALID_DEVID flag value in
>>>> kvm_irq_routing_entry to indicate the msi devid is populated.
>>>>
>>>> Signed-off-by: Eric Auger <eric.auger@redhat.com>
>>>> Reviewed-by: Andre Przywara <andre.przywara@arm.com>
>>>>
>>>> ---
>>>>  
>>>> +devid: If KVM_MSI_VALID_DEVID is set, contains a unique device identifier
>>>> +       for the device that wrote the MSI message.
>>>> +       For PCI, this is usually a BFD identifier in the lower 16 bits.
>>>> +
>>>> +The per-VM KVM_CAP_MSI_DEVID capability advertises the requirement to
>>>> +provide the device ID. If this capability is not set, userland cannot
>>>> +rely on the kernel to allow the KVM_MSI_VALID_DEVID flag being set.
>>>
>>> It would be better to enforce this mentioned dependency on set
>>> KVM_CAP_MSI_DEVID, but is the dependency even required?
>>> It seems we were checking flags for zero, so KVM_MSI_VALID_DEVID
>>> couldn't have been set by old userspaces, therefor it is ok to only make
>>> it depend only on the presence of KVM_CAP_MSI_DEVID, like the patch does
>>> now.  (I assume KVM_MSI_VALID_DEVID and KVM_CAP_MSI_DEVID are being
>>> merged at the same time.)
>>>
>>> Then there would be little point in having KVM_CAP_MSI_DEVID enableable,
>>> so does enabling KVM_CAP_MSI_DEVID mean that every MSI must have a valid
>>> devid?
>>
>> KVM_CAP_MSI_DEVID tells userland that it's fine to set the
>> KVM_MSI_VALID_DEVID flag (because the kernel would bark otherwise).
>>
>> KVM_MSI_VALID_DEVID tells the kernel that there is some meaningful
>> device ID data in the field formerly known as "pad".
>>
>> IIRC we started with the VALID_DEVID flag, then found that we need the
>> CAP because we repurposed the pad field.
>>
>> Does that make sense? Admittedly this _is_ confusing ;-)
> 
> It does, thanks.
> Some capability is need and I thought that KVM_CAP_MSI_DEVID has to be
> enabled by userspace before KVM_MSI_VALID_DEVID can be used, which isn't
> the case.  It is enabled conditionally based on vgic ITS ... my bad.
> 

Great

Thanks Andre for the clarification

Eric