Date:   Wed, 31 Mar 2021 16:14:47 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Zhang Rui' <rui.zhang@...el.com>,
        Xiaofei Tan <tanxiaofei@...wei.com>,
        "rjw@...ysocki.net" <rjw@...ysocki.net>,
        "lenb@...nel.org" <lenb@...nel.org>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>
CC:     "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: Zhang Rui
> Sent: 31 March 2021 16:55
> On Tue, 2021-03-30 at 08:14 +0000, David Laight wrote:
> > From: Zhang Rui
> > > Sent: 30 March 2021 09:00
> > > > On Tue, 2021-03-30 at 10:23 +0800, Xiaofei Tan wrote:
> > > > > Hi David,
> > > > >
> > > > > On 2021/3/29 18:09, David Laight wrote:
> > > > > > From: Xiaofei Tan
> > > > > > > Sent: 27 March 2021 07:46
> > > > > > >
> > > > > > > Replace __attribute__((packed)) by __packed following the
> > > > > > > advice of checkpatch.pl.
> > > > > > >
> > > > > > > Signed-off-by: Xiaofei Tan <tanxiaofei@...wei.com>
> > > > > > > ---
> > > > > > >  drivers/acpi/acpi_fpdt.c | 6 +++---
> > > > > > >  1 file changed, 3 insertions(+), 3 deletions(-)
> > > > > > >
> > > > > > > diff --git a/drivers/acpi/acpi_fpdt.c
> > > > > > > b/drivers/acpi/acpi_fpdt.c
> > > > > > > index a89a806..690a88a 100644
> > > > > > > --- a/drivers/acpi/acpi_fpdt.c
> > > > > > > +++ b/drivers/acpi/acpi_fpdt.c
> > > > > > > @@ -53,7 +53,7 @@ struct resume_performance_record {
> > > > > > >  	u32 resume_count;
> > > > > > >  	u64 resume_prev;
> > > > > > >  	u64 resume_avg;
> > > > > > > -} __attribute__((packed));
> > > > > > > +} __packed;
> > > > > > >
> > > > > > >  struct boot_performance_record {
> > > > > > >  	struct fpdt_record_header header;
> > > > > > > @@ -63,13 +63,13 @@ struct boot_performance_record {
> > > > > > >  	u64 bootloader_launch;
> > > > > > >  	u64 exitbootservice_start;
> > > > > > >  	u64 exitbootservice_end;
> > > > > > > -} __attribute__((packed));
> > > > > > > +} __packed;
> > > > > > >
> > > > > > >  struct suspend_performance_record {
> > > > > > >  	struct fpdt_record_header header;
> > > > > > >  	u64 suspend_start;
> > > > > > >  	u64 suspend_end;
> > > > > > > -} __attribute__((packed));
> > > > > > > +} __packed;
> > > > > >
> > > > > > My standard question about 'packed' is whether it is actually
> > > > > > needed.
> > > > > > It should only be used if the structures might be misaligned
> > > > > > in
> > > > > > memory.
> > > > > > If the only problem is that a 64bit item needs to be 32bit
> > > > > > aligned
> > > > > > then a suitable type should be used for those specific
> > > > > > fields.
> > > > > >
> > > > > > Those all look very dubious - the standard header isn't
> > > > > > packed
> > > > > > so everything must eb assumed to be at least 32bit aligned.
> > > > > >
> > > > > > There are also other sub-structures that contain 64bit
> > > > > > values.
> > > > > > These don't contain padding - but that requires 64bit
> > > > > > alignement.
> > > > > >
> > > > > > The only problematic structure is the last one - which would
> > > > > > have
> > > > > > a 32bit pad after the header.
> > > > > > Is this even right given than there are explicit alignment
> > > > > > pads
> > > > > > in some of the other structures.
> > > > > >
> > > > > > If 64bit alignment isn't guaranteed then a '64bit aligned to
> > > > > > 32bit'
> > > > > > type should be used for the u64 fields.
> > > > > >
> > > > >
> > > > > Yes, some of them has been aligned already, then nothing
> > > > > changed
> > > > > when
> > > > > add this "packed ". Maybe the purpose of the original author is
> > > > > for
> > > > > extension, and can tell others that this struct need be packed.
> > > > >
> > > >
> > > > The patch is upstreamed recently but it was made long time ago.
> > > > I think the original problem is that one of the address, probably
> > > > the
> > > > suspend_performance record, is not 64bit aligned, thus we can not
> > > > read
> > > > the proper content of suspend_start and suspend_end, mapped from
> > > > physical memory.
> > > >
> > > > I will try to find a machine to reproduce the problem with all
> > > > __attribute__((packed)) removed to double confirm this.
> > > >
> > >
> > > So here is the problem, without __attribute__((packed))
> > >
> > > [    0.858442] suspend_record: 0xffffaad500175020
> > > /sys/firmware/acpi/fpdt/suspend/suspend_end_ns:addr:
> > > 0xffffaad500175030, 15998179292659843072
> > > /sys/firmware/acpi/fpdt/suspend/suspend_start_ns:addr:
> > > 0xffffaad500175028, 0
> > >
> > > suspend_record is mapped to 0xffffaad500175020, and it is combined
> > > with
> > > one 32bit header and two 64bit fields (suspend_start and
> > > suspend_end),
> > > this is how it is located in physical memory.
> > > So the addresses of the two 64bit fields are actually not 64bit
> > > aligned.
> > >
> > > David,
> > > Is this the "a 64bit item needs to be 32bit aligned" problem you
> > > referred?
> > > If yes, what is the proper fix? should I used two 32bits for each
> > > of
> > > the field instead?
> >
> > Define something like:
> > typedef u64 __attribute__((aligned(4))) u64_align32;
> > and then use it for the 64bit structure members.
> >
> Hi, David,
> 
> Please kindly help check if the following patch is the right fix or
> not. I've verified it to work on my test box.
> 
> The reason I use this typedef for all the u64 items because there is no
> guarantee that the suspend_performance record is in the end of the
> memory, thus it may pollute the others.

Looks about right.

	David

> 
> 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.
> 
> Signed-off-by: Zhang Rui <rui.zhang@...el.com>
> ---
>  drivers/acpi/acpi_fpdt.c | 28 +++++++++++++++-------------
>  1 file changed, 15 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/acpi/acpi_fpdt.c b/drivers/acpi/acpi_fpdt.c
> index a89a806a7a2a..94e107b9a114 100644
> --- a/drivers/acpi/acpi_fpdt.c
> +++ b/drivers/acpi/acpi_fpdt.c
> @@ -23,12 +23,14 @@ enum fpdt_subtable_type {
>  	SUBTABLE_S3PT,
>  };
> 
> +typedef u64 __attribute__((aligned(4))) u64_align32;
> +
>  struct fpdt_subtable_entry {
>  	u16 type;		/* refer to enum fpdt_subtable_type */
>  	u8 length;
>  	u8 revision;
>  	u32 reserved;
> -	u64 address;		/* physical address of the S3PT/FBPT table */
> +	u64_align32 address;		/* physical address of the S3PT/FBPT table */
>  };
> 
>  struct fpdt_subtable_header {
> @@ -51,25 +53,25 @@ struct fpdt_record_header {
>  struct resume_performance_record {
>  	struct fpdt_record_header header;
>  	u32 resume_count;
> -	u64 resume_prev;
> -	u64 resume_avg;
> -} __attribute__((packed));
> +	u64_align32 resume_prev;
> +	u64_align32 resume_avg;
> +};
> 
>  struct boot_performance_record {
>  	struct fpdt_record_header header;
>  	u32 reserved;
> -	u64 firmware_start;
> -	u64 bootloader_load;
> -	u64 bootloader_launch;
> -	u64 exitbootservice_start;
> -	u64 exitbootservice_end;
> -} __attribute__((packed));
> +	u64_align32 firmware_start;
> +	u64_align32 bootloader_load;
> +	u64_align32 bootloader_launch;
> +	u64_align32 exitbootservice_start;
> +	u64_align32 exitbootservice_end;
> +};
> 
>  struct suspend_performance_record {
>  	struct fpdt_record_header header;
> -	u64 suspend_start;
> -	u64 suspend_end;
> -} __attribute__((packed));
> +	u64_align32 suspend_start;
> +	u64_align32 suspend_end;
> +};
> 
> 
>  static struct resume_performance_record *record_resume;
> --
> 2.17.1
> 

-
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