linux-kernel - RE: [PATCH v2 04/15] ACPI: table: replace __attribute__((packed)) by _

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Date:   Thu, 1 Apr 2021 08:59:12 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Bjorn Helgaas' <helgaas@...nel.org>,
        Zhang Rui <rui.zhang@...el.com>
CC:     Xiaofei Tan <tanxiaofei@...wei.com>,
        "rjw@...ysocki.net" <rjw@...ysocki.net>,
        "lenb@...nel.org" <lenb@...nel.org>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linuxarm@...neuler.org" <linuxarm@...neuler.org>
Subject: RE: [PATCH v2 04/15] ACPI: table: replace __attribute__((packed)) by
 __packed

From: Bjorn Helgaas
> Sent: 31 March 2021 18:22
> 
> On Wed, Mar 31, 2021 at 11:55:08PM +0800, Zhang Rui wrote:
> > ...
> 
> > From e18c942855e2f51e814d057fff4dd951cd0d0907 Mon Sep 17 00:00:00 2001
> > From: Zhang Rui <rui.zhang@...el.com>
> > Date: Wed, 31 Mar 2021 20:34:13 +0800
> > Subject: [PATCH] ACPI: tables: FPDT: Fix 64bit alignment issue
> >
> > Some of the 64bit items in FPDT table may be 32bit aligned.
> > Using __attribute__((packed)) is not needed in this case, fixing it by
> > allowing 32bit alignment for these 64bit items.
> 
> 1) Can you please add a spec reference for this?  I think it's ACPI
>    v6.3, sec 5.2.23.5, or something close to that.
> 
> 2) The exact layout in memory is prescribed by the spec.  I think
>    that's basically what "packed" accomplishes.  I don't understand
>    why using "aligned" would be preferable.  Using "aligned" means
>    things can be at different offsets depending on the starting
>    address of the structure.  We always want the identical layout, no
>    matter what the starting address is.

Both 'packed' and 'aligned(4)' remove any structure alignment
padding before 64bit items that aren't on an 8 byte boundary.
(Because everything else in the structures is naturally aligned.)

The difference is significant on cpu that don't support misaligned
addresses.
Assuming that the structure is always on a 4n byte boundary
(which the ACPI spec probably requires) accesses to the 32-bit
fields are always ok.
It is only 64-bit fields that must be accessed as two 32-bit
memory cycles, not all the fields using multiple single byte
cycles.

	David

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