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

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Message-ID: <BL0PR2101MB0930523DB18C6F1C1CA00A89CA739@BL0PR2101MB0930.namprd21.prod.outlook.com>
Date:   Fri, 9 Apr 2021 03:58:17 +0000
From:   Haiyang Zhang <haiyangz@...rosoft.com>
To:     David Miller <davem@...emloft.net>,
        Dexuan Cui <decui@...rosoft.com>
CC:     "kuba@...nel.org" <kuba@...nel.org>,
        KY Srinivasan <kys@...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)



> -----Original Message-----
> From: David Miller <davem@...emloft.net>
> Sent: Thursday, April 8, 2021 8:41 PM
> To: Dexuan Cui <decui@...rosoft.com>
> Cc: kuba@...nel.org; KY Srinivasan <kys@...rosoft.com>; Haiyang Zhang
> <haiyangz@...rosoft.com>; Stephen Hemminger
> <sthemmin@...rosoft.com>; wei.liu@...nel.org; Wei Liu
> <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.

For the structs containing variables with the same sizes, or already size aligned 
variables, we knew the __packed has no effect. And for these structs, it doesn't 
cause performance impact either, correct? 

But in the future, if different sized variables are added, the __packed may 
become necessary again. To prevent anyone accidently forget to add __packed 
when adding new variables to these structs, can we keep the __packed for all 
messages going through the "wire"?

Thanks,
- Haiyang