Re: [PATCH 2/2] xen/dm: arm: Introduce inject_msi2 DM op

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.03.2025 14:31, Mykyta Poturai wrote:
> On 18.03.25 12:11, Jan Beulich wrote:
>> On 18.03.2025 10:10, Mykyta Poturai wrote:
>>> On 15.01.24 11:35, Jan Beulich wrote:
>>>> On 14.01.2024 11:01, Mykyta Poturai wrote:
>>>>> --- a/xen/include/public/hvm/dm_op.h
>>>>> +++ b/xen/include/public/hvm/dm_op.h
>>>>> @@ -444,6 +444,17 @@ struct xen_dm_op_nr_vcpus {
>>>>>    };
>>>>>    typedef struct xen_dm_op_nr_vcpus xen_dm_op_nr_vcpus_t;
>>>>>
>>>>> +#define XEN_DMOP_inject_msi2 21
>>>>> +#define XEN_DMOP_MSI_SOURCE_ID_VALID (1u << 0)
>>>>> +
>>>>> +struct xen_dm_op_inject_msi2 {
>>>>> +    uint64_aligned_t addr;
>>>>> +    uint32_t data;
>>>>> +    uint32_t source_id; /* PCI SBDF */
>>>>
>>>> Since the comment says SBDF (not BDF), how are multiple segments handled
>>>> here? At least on x86 (VT-d and V-i) source IDs are only 16 bits iirc,
>>>> and are local to the respective segment. It would feel wrong to use a
>>>> 32-bit quantity there; IOW I'd rather see this as being two 16-bit fields
>>>> (segment and source_id).
>>>
>>> I'm planning on resuming this series in the near future and want to
>>> clarify the DM op interface.
>>>
>>> Wouldn't it be better to keep things consistent and use
>>> XEN_DMOP_PCI_SBDF as it's done in xen_dm_op_ioreq_server_range? Also
>>> with this, the value can be easily casted to pci_sbdf_t later and split
>>> to segment and BDF if needed.
>>
>> The essence of my earlier comment is: Naming, contents, and comments need
>> to be in sync.
>>
>> I question though that "casting to pci_sbdf_t" is technically possible.
>> Nor am I convinced that it would be desirable to do so if it was possible
>> (or if "casting" was intended to mean something else than what this is in
>> C). See my recent comments on some of Andrii's patches [1][2].
>>
>> Jan
>>
>> [1] https://lists.xen.org/archives/html/xen-devel/2025-03/msg01294.html
>> [2] https://lists.xen.org/archives/html/xen-devel/2025-03/msg01301.html
> 
> Would something like this be okay then?
> 
> struct xen_dm_op_inject_msi2 {
>      /* IN - MSI data (lower 32 bits) */
>      uint32_t data;
>      /* IN - ITS devid of the device triggering the interrupt */
>      uint32_t source_id;
>      uint32_t flags;
>      uint32_t pad;
>      /* IN - MSI address */
>      uint64_aligned_t addr;
> };
> 
> Added padding and explained source_id purpose better.

I fear the comment is far from making clear what layout source_id is to
have, hence also leaving open whether a segment number is included there.
Of course the issue here may be that I have no clue what "ITS devid"
means or implies. What is clear is that "ITS devid" is meaningless on
x86, for example.

I'm further puzzled by "(lower 32 bits)" - isn't the data written to
trigger an MSI always a 32-bit quantity?

Jan