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

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Date:   Tue, 30 Mar 2021 15:31:19 +0800
From:   Zhang Rui <rui.zhang@...el.com>
To:     Xiaofei Tan <tanxiaofei@...wei.com>,
        David Laight <David.Laight@...LAB.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

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.

thanks,
rui
> > 	David
> > 
> > -
> > Registered Address Lakeside, Bramley Road, Mount Farm, Milton
> > Keynes, MK1 1PT, UK
> > Registration No: 1397386 (Wales)
> > 
> > 
> > .
> > 
> 
>