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Message-ID: <e7c5f4334ab6ff897547c68ea216fbcba22d4929.camel@suse.com>
Subject: Re: [PATCH] scsi: Let scsi_execute_cmd() mark args->sshdr as invalid
From:   Martin Wilck <mwilck@suse.com>
To:     Juergen Gross <jgross@suse.com>, linux-kernel@vger.kernel.org,
        linux-scsi@vger.kernel.org
Cc:     "James E.J. Bottomley" <jejb@linux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        stable@vger.kernel.org
Date:   Thu, 11 May 2023 15:23:14 +0200
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         <095a2264120ad51d0500c4ce8221be2f88a9537e.camel@suse.com>
         <85a7dc28-74ec-f4d6-b5c3-ca456ce9d380@suse.com>
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On Thu, 2023-05-11 at 15:17 +0200, Juergen Gross wrote:
> >=20
> > We know for certain that sizeof(*sshdr) is 8 bytes, and will most
> > probably remain so. Thus
> >=20
> > =A0=A0=A0=A0 memset(sshdr, 0, sizeof(*sshdr))
> >=20
> > would result in more efficient code.
>=20
> 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) [FT=
RACE NOP]
0xffffffff8177e9d5 <scsi_normalize_sense+5>:    test   %rdi,%rdi
0xffffffff8177e9d8 <scsi_normalize_sense+8>:    movq   $0x0,(%rdx)

Martin