Re: [PATCH 3/5] RDMA/srp: Apply the __packed attribute to members instead of structures

[Bart Van Assche <bvanassche@acm.org>]

On 5/20/21 7:48 AM, Jason Gunthorpe wrote:
> On Tue, May 11, 2021 at 08:27:50PM -0700, Bart Van Assche wrote:
>> @@ -107,10 +107,10 @@ struct srp_direct_buf {
>>   * having the 20-byte structure padded to 24 bytes on 64-bit architectures.
>>   */
>>  struct srp_indirect_buf {
>> -	struct srp_direct_buf	table_desc;
>> +	struct srp_direct_buf	table_desc __packed;
>>  	__be32			len;
>> -	struct srp_direct_buf	desc_list[];
>> -} __attribute__((packed));
>> +	struct srp_direct_buf	desc_list[] __packed;
>> +};
>>  
>>  /* Immediate data buffer descriptor as defined in SRP2. */
>>  struct srp_imm_buf {
>> @@ -175,13 +175,13 @@ struct srp_login_rsp {
>>  	u8	opcode;
>>  	u8	reserved1[3];
>>  	__be32	req_lim_delta;
>> -	u64	tag;
>> +	u64	tag __packed;
> 
> What you really want is just something like this:
> 
> typedef u64 __attribute__((aligned(4))) compat_u64;
> 
> And then use that every place you have the u64 and forget about packed
> entirely.

Really? My understanding is that the aligned attribute can be used to
increase alignment of a structure member but not to decrease it. From
https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#Common-Variable-Attributes:
"When used on a struct, or struct member, the aligned attribute can only
increase the alignment; in order to decrease it, the packed attribute
must be specified as well."

Additionally, there is a recommendation in
Documentation/process/coding-style.rst not to introduce new typedefs.

> Except for a couple exceptions IBA mads are always aligned to 4 bytes,
> only the 64 bit quantities are unaligned.
> 
> But really this whole thing should be replaced with the IBA_FIELD
> macros like include/rdma/ibta_vol1_c12.h demos.
> 
> Then it would be sparse safe and obviously endian correct as well.

I prefer a structure over the IBA_FIELD macros so I will change __packed
into __packed __aligned(4).

Thanks,

Bart.