[L0] Backend context will contain all devices even if frontend context contains single device

[CaoZhongZ]

Which will let the driver create extra allocations in certain scenarios. Should we use zeContextCreateEx instead of zeContextCreate?

https://github.com/intel/llvm/blob/ad880488f093333d93e4025573587f7d522e79d5/sycl/plugins/unified_runtime/ur/adapters/level_zero/context.cpp#L32

[jandres742]

@CaoZhongZ : the devices exposed by the L0 driver vary depending on ZE_AFFINITY_MASK, while the devices exposed by the SYCL runtime are determined by ONEAPI_DEVICE_SELECTOR. Idea is that a SYCL user uses the ONEAPI_DEVICE_SELECTOR to select the devices that wants to use, and SYCL runtime filters those from all available from L0 (or the subset defined by ZE_AFFINITY_MASK).

Now, we could definitely make this safer by creating the L0 context only with a sub-set of the devices with the function you mentioned above. So far, we haven't gotten reports of issues (that I recall) that are not working correctly due to this, which is why we may have not added the support.

If you have a failed case, then we can prioritize adding this.

CC: @smaslov-intel , to confirm my understanding above.

[CaoZhongZ]

Yes, the safer part is what I mean. We triggered other problem in backend, though it could be fixed but the performance would also be impaired a little bit.

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