Re: [PATCH v4 1/1] scsi: ufs: core: add device level exception support

["Bao D. Nguyen" <quic_nguyenb@quicinc.com>]

On 3/27/2025 4:36 AM, Bart Van Assche wrote:
> On 3/26/25 7:47 PM, Bao D. Nguyen wrote:
>> On 3/26/2025 3:49 AM, Bart Van Assche wrote:
>>> On 3/25/25 6:15 PM, Bao D. Nguyen wrote:
>>>> The existing "struct utp_upiu_query_v4_0" probably has a bug in it. 
>>>> It does not use the  __attribute__((__packed__)) attribute. The 
>>>> compiler is free to add padding in this structure, resulting in the 
>>>> read attribute value being incorrect. I plan to provide a separate 
>>>> patch to fix this issue.
>>>
>>> Adding __attribute__((__packed__)) or __packed to data structures that
>>> don't need it is not an improvement but is a change that makes
>>> processing slower on architectures that do not support unaligned
>>> accesses. Instead of adding __packed to data structures in their
>>> entirety, only add it to those members that need it and check the
>>> structure size as follows:
>>>
>>> static_assert(sizeof(...) == ...);
>>>
>> Thank you for the info on this, Bart.
>> IMO, this response upiu data should be __packed because the data 
>> coming from the hardware follows a strict format as defined by the 
>> spec. If we support __pack each individual field which data may be 
>> read by the driver (the attribute read commands) and check the 
>> validity of their sizes, it may add some complexity?
> 
> Hi Bao,
> 
> As explained in my previous email, adding __packed to data structures in
> their entirety is a bad practice. Please don't do this.
> 
> Regarding your question: I have not yet seen any data structure that
> represents an on-the-wire data format where every single data member
> has to be annotated with __packed. Only data members that are not
> aligned to a natural boundary need this annotation. Examples are
> available in this header file: include/scsi/srp.h.
> 

Thanks Bart. How about we change the current utp_upiu_query_v4_0 to

struct utp_upiu_query_v4_0 {
         __u8 opcode;
         __u8 idn;
         __u8 index;
         __u8 selector;
         __u8 cmd_specifics[8];
         /* private: */
         __be32 reserved;
};

Depending on the opcode/transaction, the cmd_specifics[] can be type 
casted to access the LENGTH, FLAG_VALUE, VALUE[0:63] fields of the QUERY 
UPIU. The __u8 array[8] would also prevent the compiler padding to the data.

Thanks, Bao