Re: [PATCH] scsi: Let scsi_execute_cmd() mark args->sshdr as invalid

[Martin Wilck]

On Thu, 2023-05-11 at 15:32 +0200, Juergen Gross wrote:
> On 11.05.23 15:23, Martin Wilck wrote:
> > On Thu, 2023-05-11 at 15:17 +0200, Juergen Gross wrote:
> > > > 
> > > > We know for certain that sizeof(*sshdr) is 8 bytes, and will
> > > > most
> > > > probably remain so. Thus
> > > > 
> > > >       memset(sshdr, 0, sizeof(*sshdr))
> > > > 
> > > > would result in more efficient code.
> > > 
> > > I fail to see why zeroing a single byte would be less efficient
> > > than
> > > zeroing
> > > a possibly unaligned 8-byte area.
> > 
> > I don't think it can be unaligned. gcc seems to think the same. It
> > compiles the memset(sshdr, ...) in scsi_normalize_sense() into a
> > single
> > instruction on x86_64.
> > 
> > 0xffffffff8177e9d0 <scsi_normalize_sense>:      nopl  
> > 0x0(%rax,%rax,1) [FTRACE NOP]
> > 0xffffffff8177e9d5 <scsi_normalize_sense+5>:    test   %rdi,%rdi
> > 0xffffffff8177e9d8 <scsi_normalize_sense+8>:    movq   $0x0,(%rdx)
> 
> A struct with 8 "u8" fields can be unaligned.

Right. I wrongly assumed this would be aligned like an u64. "The
alignment of any given struct or union type is required by the ISO C
standard to be at least a perfect multiple of the lowest common
multiple of the alignments of all of the members of the struct".

I wonder if this (non-)alignment of struct scsi_sense_hdr is
intentional, but that's a different discussion.

Thanks,
Martin