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

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Message-ID: <20220706163434.GA2222@lst.de>
Date:   Wed, 6 Jul 2022 18:34:34 +0200
From:   Christoph Hellwig <hch@....de>
To:     Keith Busch <kbusch@...nel.org>
Cc:     Christoph Hellwig <hch@....de>, John Garry <john.garry@...wei.com>,
        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 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. 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().