Re: [PATCH-for-6.0 v2 25/25] block/nvme: Simplify Completion Queue Command Identifier field use

[Philippe Mathieu-Daudé]

On 10/30/20 3:00 PM, Stefan Hajnoczi wrote:
> On Thu, Oct 29, 2020 at 10:33:06AM +0100, Philippe Mathieu-Daudé wrote:
>> The "Completion Queue Entry: DW 2" describes it as:
>>
>>   This identifier is assigned by host software when
>>   the command is submitted to the Submission
>>
>> As the is just an opaque cookie, it is pointless to byte-swap it.
>>
>> Suggested-by: Keith Busch <kbusch@kernel.org>
>> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
>> ---
>>  block/nvme.c | 6 +++---
>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/block/nvme.c b/block/nvme.c
>> index a06a188d530..e7723c42a6d 100644
>> --- a/block/nvme.c
>> +++ b/block/nvme.c
>> @@ -344,7 +344,7 @@ static inline int nvme_translate_error(const NvmeCqe *c)
>>          trace_nvme_error(le32_to_cpu(c->result),
>>                           le16_to_cpu(c->sq_head),
>>                           le16_to_cpu(c->sq_id),
>> -                         le16_to_cpu(c->cid),
>> +                         c->cid,
>>                           le16_to_cpu(status));
>>      }
>>      switch (status) {
>> @@ -401,7 +401,7 @@ static bool nvme_process_completion(NVMeQueuePair *q)
>>          if (!q->cq.head) {
>>              q->cq_phase = !q->cq_phase;
>>          }
>> -        cid = le16_to_cpu(c->cid);
>> +        cid = c->cid;
>>          if (cid == 0 || cid > NVME_QUEUE_SIZE) {
>>              warn_report("NVMe: Unexpected CID in completion queue: 
>> %"PRIu32", "
>>                          "queue size: %u", cid, NVME_QUEUE_SIZE);
>> @@ -469,7 +469,7 @@ static void nvme_submit_command(NVMeQueuePair *q, 
>> NVMeRequest *req,
>>      assert(!req->cb);
>>      req->cb = cb;
>>      req->opaque = opaque;
>> -    cmd->cid = cpu_to_le16(req->cid);
>> +    cmd->cid = req->cid;
>>  
>>      trace_nvme_submit_command(q->s, q->index, req->cid);
>>      nvme_trace_command(cmd);
> 
> Eliminating the byteswap is safe but this patch makes the code
> confusing, as I mentioned previously.
> 
> Please use a comment or macro to mark this field native endian. It's not
> obvious to the reader that we can skip the byteswap here.
> 
> Otherwise it will confuse readers into adding the byteswap back, not
> using byteswapping in other places where it is needed, etc.

OK. (This patch is for 6.0 anyway, I included because it was
following the previous patch in its previous version).

> 
> Thanks,
> Stefan
>