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Re: [PATCH 1/2] xennet: improve NBL batching receive path
- To: Tu Dinh <ngoc-tu.dinh@xxxxxxxxxx>, David ambu <david.preetham@xxxxxxxxxx>, "win-pv-devel@xxxxxxxxxxxxxxxxxxxx" <win-pv-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Owen Smith <owen.smith@xxxxxxxxxx>
- Date: Wed, 23 Sep 2026 09:53:10 +0000
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- Thread-topic: [PATCH 1/2] xennet: improve NBL batching receive path
>>
>> - Returned = Receiver->Returned;
>> + (VOID) InterlockedAdd(&Receiver->Indicated, Count);
>>
>> KeMemoryBarrier();
>>
>> + Returned = Receiver->Returned;
>> Indicated = Receiver->Indicated;
>
>I wasn't sure about the purpose of the previous barriers so I went
>digging. I've found that Receiver->Returned and Receiver->Indicated are
>only ever added to, without a dependency between the two. So wouldn't
>this be sufficient without any further barriers?
>
> if (Count == 0)
> return;
>
> Indicated = InterlockedAdd(&Receiver->Indicated, Count);
> Returned = Receiver->Returned;
I was also looking into this, as the InterlockedAdd operation does a full
memory barrier,
KeMemoryBarrier is not required. I'd put the Returned read before the
InterlockedAdd,
so its sampled before the implicit barrier.
I think there is also an improvement here, and the caller, where the
Queue->Lock can be acquired
once - to insert the NBL, and if Push, swap out the NBL - but this should be
looked at as a
future change
Owen
>
>>
>> Flags = NDIS_RECEIVE_FLAGS_DISPATCH_LEVEL |
>
>--
>Ngoc Tu Dinh | Vates XCP-ng Developer
>
>XCP-ng & Xen Orchestra - Vates solutions
web: https://vates.tech
|
