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

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Date:   Thu, 08 Apr 2021 17:41:22 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     decui@...rosoft.com
Cc:     kuba@...nel.org, kys@...rosoft.com, haiyangz@...rosoft.com,
        sthemmin@...rosoft.com, wei.liu@...nel.org, liuwe@...rosoft.com,
        netdev@...r.kernel.org, leon@...nel.org, andrew@...n.ch,
        bernd@...rovitsch.priv.at, rdunlap@...radead.org,
        linux-kernel@...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: Dexuan Cui <decui@...rosoft.com>
Date: Fri, 9 Apr 2021 00:24:51 +0000

>> 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?

It actually isan't in many cases, check with and without the __packed directive
and see if anything chasnges.

> 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);

This won't make sure the structure member offsets are what you expect.

I think you'll have to go through the structures one-by-one by hand to
figure out which ones really require the __packed attribute and which do not.

Thank you.