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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 9/25/24 12:09, Kanchan Joshi wrote: > On 9/25/2024 11:27 AM, Hannes Reinecke wrote: ... > As it stands the new struct will introduce >> a hole of 24 bytes after 'hint_type'. > > This gets implicitly padded at this point [1][2], and overall size is > still capped by largest struct (which is of 16 bytes, placed just above > this). For me it's about having hardly usable in the future by anyone else 7 bytes of space or how much that will be. Try to add another field using those bytes and endianess will start messing with you. And 7 bytes is not that convenient. I have same problem with how commands were merged while I was not looking. There was no explicit padding, and it split u64 into u32 and implicit padding, so no apps can use the space to put a pointer anymore while there was a much better option of using one of existing 4B fields. > [1] On 64bit > » union { > » » struct { > » » » __u64 addr3; /* 48 8 */ > » » » __u64 __pad2[1]; /* 56 8 */ > » » }; /* 48 16 */ > » » struct { > » » » __u64 hint_val; /* 48 8 */ > » » » __u8 hint_type; /* 56 1 */ > » » }; /* 48 16 */ > » » __u64 optval; /* 48 8 */ > » » __u8 cmd[0]; /* 48 0 */ > » }; /* 48 16 */ > > » /* size: 64, cachelines: 1, members: 13 */ > > [2] On 32bit > > » union { > » » struct { > » » » __u64 addr3; /* 48 8 */ > » » » __u64 __pad2[1]; /* 56 8 */ > » » }; /* 48 16 */ > » » struct { > » » » __u64 hint_val; /* 48 8 */ > » » » __u8 hint_type; /* 56 1 */ > » » }; /* 48 12 */ > » » __u64 optval; /* 48 8 */ > » » __u8 cmd[0]; /* 48 0 */ > » }; /* 48 16 */ > > » /* size: 64, cachelines: 1, members: 13 */ > }; -- Pavel Begunkov