linux-kernel - Re: [PATCH] nvme: Fix nvme_setup_command metadata trace event for cdw10

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Date:   Wed, 6 Jul 2022 17:58:49 +0100
From:   John Garry <john.garry@...wei.com>
To:     Keith Busch <kbusch@...nel.org>, Christoph Hellwig <hch@....de>
CC:     <axboe@...com>, <sagi@...mberg.me>,
        <linux-nvme@...ts.infradead.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] nvme: Fix nvme_setup_command metadata trace event for
 cdw10

On 06/07/2022 17:44, Keith Busch wrote:
> On Wed, Jul 06, 2022 at 06:34:34PM +0200, Christoph Hellwig wrote:
>> On Wed, Jul 06, 2022 at 10:26:09AM -0600, Keith Busch wrote:
>>> On Wed, Jul 06, 2022 at 06:18:25PM +0200, Christoph Hellwig wrote:
>>>> On Wed, Jul 06, 2022 at 10:13:22AM -0600, Keith Busch wrote:
>>>>> Did you test what the trace looks like afte this? We're losing valuable trace
>>>>> data here. The field is supposed to get CDW's 10 - 15, so that's 24 bytes.

ok, I just thought it was a typo, but did not know why you were using an 
array macro.

> I
>>>>> don't know why it cares that the address of the field being read is only 4
>>>>> bytes; we want everything that comes after it too.
>>>>
>>>> Because accesses should not spawn boundaries of members in structs unless
>>>> copying the entire struct.  If we want to trace the various fields we
>>>> need to individually assign them.
>>>>
>>>> Anyway, I'm dropping this patch from nvme-5.19 for now to let the
>>>> discussion conclude.
>>>
>>> How about this instead?
>>
>> Maybe a better option would be to use struct_group().
> 
> Good call, I'd never used that macro before. The result produces anonymous
> unions like I just proposed, so yes, I like that option.
> .

The warning hints at using struct_group() also ...

Anyway, Keith, do you want to write a new patch or shall I?

Thanks,
John