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@xxxxxxxxxx>
>> Reviewed-by: Andre Przywara <andre.przywara@xxxxxxx>
>>
>> ---
>>
>> v6 -> v7:
>> - Added Andre's R-b
>>
>> v4 -> v5:
>> - some rephrasing in api.txt according to Christoffer's comments
>> v2 -> v3:
>> - replace usage of KVM_IRQ_ROUTING_EXTENDED_MSI type by
>> usage of KVM_MSI_VALID_DEVID flag
>> - add note about KVM_CAP_MSI_DEVID capability
>>
>> v1 -> v2:
>> - devid id passed in kvm_irq_routing_msi instead of in
>> kvm_irq_routing_entry
>>
>> RFC -> PATCH
>> - remove kvm_irq_routing_extended_msi and use union instead
>> ---
>> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
>> @@ -1479,9 +1483,20 @@ struct kvm_irq_routing_msi {
>> __u32 address_lo;
>> __u32 address_hi;
>> __u32 data;
>> - __u32 pad;
>> + union {
>> + __u32 pad;
>> + __u32 devid;
>> + };
>> };
>>
>> +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 ;-)
Cheers,
Andre.
>
> Thanks.
>
> ---
> I'm confused about the purpose behind two dynamic flags that seem to do
> that same thing, but those are just nitpicks, the API looks good in
> general.
>
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