linux-kernel - RE: [PATCH v3 net-next] net: mana: Add a driver for Microsoft Azure Network Adapter (MANA)

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Date:   Fri, 9 Apr 2021 00:24:51 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     David Miller <davem@...emloft.net>
CC:     "kuba@...nel.org" <kuba@...nel.org>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        Wei Liu <liuwe@...rosoft.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "leon@...nel.org" <leon@...nel.org>,
        "andrew@...n.ch" <andrew@...n.ch>,
        "bernd@...rovitsch.priv.at" <bernd@...rovitsch.priv.at>,
        "rdunlap@...radead.org" <rdunlap@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>
Subject: RE: [PATCH v3 net-next] net: mana: Add a driver for Microsoft Azure
 Network Adapter (MANA)

> From: David Miller <davem@...emloft.net>
> Sent: Thursday, April 8, 2021 4:46 PM
> ...
> > +struct gdma_msg_hdr {
> > +	u32 hdr_type;
> > +	u32 msg_type;
> > +	u16 msg_version;
> > +	u16 hwc_msg_id;
> > +	u32 msg_size;
> > +} __packed;
> > +
> > +struct gdma_dev_id {
> > +	union {
> > +		struct {
> > +			u16 type;
> > +			u16 instance;
> > +		};
> > +
> > +		u32 as_uint32;
> > +	};
> > +} __packed;
> 
> Please don't  use __packed unless absolutely necessary.  It generates
> suboptimal code (byte at a time
> accesses etc.) and for many of these you don't even need it.

In the driver code, all the structs/unions marked by __packed are used to
talk with the hardware, so I think __packed is necessary here?

Do you think if it's better if we remove all the __packed, and add
static_assert(sizeof(struct XXX) == YYY) instead? e.g.

@@ -105,7 +105,8 @@ struct gdma_msg_hdr {
        u16 msg_version;
        u16 hwc_msg_id;
        u32 msg_size;
-} __packed;
+};
+static_assert(sizeof(struct gdma_msg_hdr) == 16);

 struct gdma_dev_id {
        union {

Now I'm trying to figure out how other NIC drivers define structs/unions.

Thanks,
Dexuan