Re: [PATCH v3 net-next 6/7] net: ena: Add error bound to PHC communication layer

[Paolo Abeni <pabeni@redhat.com>]

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---
net: ena: Add error bound to PHC communication layer

This commit extends the ENA PHC communication layer to retrieve an
error_bound value from the device alongside the timestamp, and adds a
phc_err_eb statistic to count error_bound retrieval failures.

> diff --git a/drivers/net/ethernet/amazon/ena/ena_com.c b/drivers/net/ethernet/amazon/ena/ena_com.c
> index 257910460bd8..832c4c1d0e5e 100644
> --- a/drivers/net/ethernet/amazon/ena/ena_com.c
> +++ b/drivers/net/ethernet/amazon/ena/ena_com.c

[ ... ]

> @@ -1882,14 +1888,15 @@ int ena_com_phc_get_timestamp(struct ena_com_dev *ena_dev, u64 *timestamp)
>  		}
>  
>  		/* req_id was updated by the device which indicates that
> -		 * PHC timestamp and error_flags are updated too,
> -		 * checking errors before retrieving timestamp
> +		 * PHC timestamp, error_bound and error_flags are updated too,
> +		 * checking error flags before retrieving timestamp and
> +		 * error_bound values
>  		 */
>  		if (unlikely(resp->error_flags & ENA_PHC_ERROR_FLAGS)) {

[High]

Once req_id is observed equal to phc->req_id, the function reads
resp->error_flags, resp->timestamp and (now) resp->error_bound from the
device-shared DMA buffer. The updated comment explicitly states that once
req_id is observed updated, "PHC timestamp, error_bound and error_flags
are updated too" — which is the producer/consumer pattern that normally
requires a read barrier between the index check and the dependent data
loads.

The buffer is declared volatile and the req_id check uses READ_ONCE(),
but on weakly-ordered architectures such as arm64 (which includes AWS
Graviton, a primary ENA deployment target) neither volatile nor
READ_ONCE() orders dependent loads against DMA writes from the device.
Without a dma_rmb() the CPU may observe the new req_id while still
returning stale or torn values for the 64-bit timestamp, the error_bound,
or the error_flags.

Could a dma_rmb() be added between the req_id check and the subsequent
reads of resp->error_flags / resp->timestamp / resp->error_bound, similar
to the pattern already used elsewhere in the same driver?

For example, ena_com_handle_admin_completion() in the same file does:

	/* Do not read the rest of the completion entry before the
	 * phase bit was validated
	 */
	dma_rmb();

and ena_com_aenq_intr_handler() has:

	/* Make sure the phase bit (ownership) is as expected before
	 * reading the rest of the descriptor.
	 */
	dma_rmb();

The same shape applies here: device writes data fields, then writes
req_id last; driver checks req_id, then reads data fields.

This ordering issue is not introduced by this patch — the timestamp and
error_flags reads after the req_id check existed before — but the patch
extends the same unsynchronized read pattern to a third field
(error_bound) and the new comment now documents the assumed ordering
that is not actually enforced. Would it make sense to address the missing
dma_rmb() while touching this code?

>  			/* Retrieved invalid PHC timestamp, PHC enters into

[ ... ]

> @@ -1897,12 +1904,15 @@ int ena_com_phc_get_timestamp(struct ena_com_dev *ena_dev, u64 *timestamp)
>  
>  		/* PHC timestamp value is returned to the caller */
>  		*timestamp = resp->timestamp;
> +		if (error_bound)
> +			*error_bound = resp->error_bound;
>  
>  		/* Update statistic on valid PHC timestamp retrieval */
>  		phc->stats.phc_cnt++;

[ ... ]
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