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

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Message-ID: <YsW3oXcCe6/y6iRb@kbusch-mbp.dhcp.thefacebook.com>
Date:   Wed, 6 Jul 2022 10:26:09 -0600
From:   Keith Busch <kbusch@...nel.org>
To:     Christoph Hellwig <hch@....de>
Cc:     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 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?

---
diff --git a/drivers/nvme/host/trace.h b/drivers/nvme/host/trace.h
index b5f85259461a..3c5e7fa03707 100644
--- a/drivers/nvme/host/trace.h
+++ b/drivers/nvme/host/trace.h
@@ -69,7 +69,7 @@ TRACE_EVENT(nvme_setup_cmd,
 		__entry->metadata = !!blk_integrity_rq(req);
 		__entry->fctype = cmd->fabrics.fctype;
 		__assign_disk_name(__entry->disk, req->q->disk);
-		memcpy(__entry->cdw10, &cmd->common.cdw10,
+		memcpy(__entry->cdw10, &cmd->common.bytes,
 			sizeof(__entry->cdw10));
 	    ),
 	    TP_printk("nvme%d: %sqid=%d, cmdid=%u, nsid=%u, flags=0x%x, meta=0x%x, cmd=(%s %s)",
diff --git a/include/linux/nvme.h b/include/linux/nvme.h
index e3934003f239..1be226871763 100644
--- a/include/linux/nvme.h
+++ b/include/linux/nvme.h
@@ -906,12 +906,17 @@ struct nvme_common_command {
 	__le32			cdw2[2];
 	__le64			metadata;
 	union nvme_data_ptr	dptr;
-	__le32			cdw10;
-	__le32			cdw11;
-	__le32			cdw12;
-	__le32			cdw13;
-	__le32			cdw14;
-	__le32			cdw15;
+	union {
+		struct {
+			__le32	cdw10;
+			__le32	cdw11;
+			__le32	cdw12;
+			__le32	cdw13;
+			__le32	cdw14;
+			__le32	cdw15;
+		};
+		__u8 bytes[24];
+	};
 };
 
 struct nvme_rw_command {
--