From: Fenghua Yu <fenghua.yu@intel.com>
To: Fenghua Yu <fenghua.yu@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Dave Hansen <dave.hansen@intel.com>,
Ingo Molnar <mingo@redhat.com>, H Peter Anvin <hpa@zytor.com>,
Ashok Raj <ashok.raj@intel.com>, Alan Cox <alan@linux.intel.com>,
Peter Zijlstra <peterz@infradead.org>,
Rafael Wysocki <rafael.j.wysocki@intel.com>,
Tony Luck <tony.luck@intel.com>,
Ravi V Shankar <ravi.v.shankar@intel.com>,
linux-kernel <linux-kernel@vger.kernel.org>, x86 <x86@kernel.org>
Subject: Re: [PATCH v2 2/4] x86/split_lock: Align x86_capability to unsigned long to avoid split locked access
Date: Fri, 29 Jun 2018 17:14:11 -0700 [thread overview]
Message-ID: <20180630001411.GD68178@romley-ivt3.sc.intel.com> (raw)
In-Reply-To: <20180630000051.GC68178@romley-ivt3.sc.intel.com>
On Fri, Jun 29, 2018 at 05:00:51PM -0700, Fenghua Yu wrote:
> On Fri, Jun 29, 2018 at 11:44:44PM +0200, Thomas Gleixner wrote:
> > On Fri, 29 Jun 2018, Dave Hansen wrote:
> >
> > > On 06/29/2018 01:38 PM, Fenghua Yu wrote:
> > > > How to handle data that is used in generic code which can be used on
> > > > non-Intel platform? For exmple, if I do this change for struct efi in
> > > > include/linux/efi.h because set_bit() sets bits in efi.flags:
> > > > - unsigned long flags;
> > > > + unsigned long flags __aligned(unsigned long);
> > > > } efi;
> > > >
> > > > People may argue that the alignment unnecessarily increases size of 'efi'
> > > > on non-Intel platform which doesn't have split lock issue. Do we care this
> > > > argument?
> > >
> > > Unaligned memory accesses are bad, pretty much universally. This is a
> > > general good practice that we should have been doing anyway. Let folks
> > > complain. Don't let it stop you.
> > >
> > > Also, look at the size of that structure. Look at how many pointers it
> > > has. Do you think *anyone* is going to complain about an extra 4 bytes
> > > in a 400-byte structure?
> >
> > But in the above case the compiler does already the right thing. Why?
> > Because struct members are aligned to their natural alignment unless the
> > struct is explicitely marked 'packed'. In that case the programmer has to
> > take care of the alignment.
> >
> > Just look at it with pahole:
> >
> > struct efi_memory_map memmap; /* 280 56 */
> >
> > /* XXX last struct has 7 bytes of padding */
> >
> > /* --- cacheline 5 boundary (320 bytes) was 16 bytes ago --- */
> > long unsigned int flags; /* 336 8 */
> >
> > The issue with the capability arrays is that the data type is u32 which has
> > the natural alignment of 4 byte, while unsigned long has 8 byte on 64bit.
> >
> > So just slapping blindly aligned(unsigned long) to anything which is
> > accessed by locked instructions is pointless.
> >
>
> Thank you for you education!
>
> Below is part of the future patches that are supposed to fix more potential
> split lock issues.
>
> Could you please take a look and see if the changes are in the
> right direction before I move further?
>
> diff --git a/arch/x86/boot/cpuflags.h b/arch/x86/boot/cpuflags.h
Please ignore the patch in my last email because of some obvious stupid
mistakes. Sorry about that.
Instead, could you please take a look at the following patch and see if
the changes are in the right direction before I move further?
diff --git a/arch/x86/boot/cpuflags.h b/arch/x86/boot/cpuflags.h
index 2e20814d3ce3..29de0ff74351 100644
--- a/arch/x86/boot/cpuflags.h
+++ b/arch/x86/boot/cpuflags.h
@@ -9,7 +9,7 @@ struct cpu_features {
int level; /* Family, or 64 for x86-64 */
int family; /* Family, always */
int model;
- u32 flags[NCAPINTS];
+ u32 flags[NCAPINTS] __aligned(sizeof(unsigned long));
};
extern struct cpu_features cpu;
diff --git a/arch/x86/include/asm/mce.h b/arch/x86/include/asm/mce.h
index 8c7b3e5a2d01..444a2275c1f8 100644
--- a/arch/x86/include/asm/mce.h
+++ b/arch/x86/include/asm/mce.h
@@ -133,7 +133,7 @@ struct mce_log_buffer {
char signature[12]; /* "MACHINECHECK" */
unsigned len; /* = MCE_LOG_LEN */
unsigned next;
- unsigned flags;
+ unsigned flags __aligned(sizeof(unsigned long));
unsigned recordlen; /* length of struct mce */
struct mce entry[MCE_LOG_LEN];
};
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index eb4cb3efd20e..e6a28163e905 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -488,8 +488,8 @@ static const char *table_lookup_model(struct cpuinfo_x86 *c)
return NULL; /* Not found */
}
-__u32 cpu_caps_cleared[NCAPINTS + NBUGINTS];
-__u32 cpu_caps_set[NCAPINTS + NBUGINTS];
+__u32 cpu_caps_cleared[NCAPINTS + NBUGINTS] __aligned(sizeof(unsigned long));
+__u32 cpu_caps_set[NCAPINTS + NBUGINTS] __aligned(sizeof(unsigned long));
void load_percpu_segment(int cpu)
{
Thanks.
-Fenghua
next prev parent reply other threads:[~2018-06-30 0:14 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-06-29 14:33 [PATCH v2 0/4] x86/split_lock: Enable #AC exception for split locked accesses Fenghua Yu
2018-06-29 14:33 ` [PATCH v2 1/4] x86/split_lock: Enumerate #AC exception for split locked access feature Fenghua Yu
2018-06-29 14:56 ` Dave Hansen
2018-06-29 16:23 ` Thomas Gleixner
2018-06-29 16:32 ` Dave Hansen
2018-07-04 20:07 ` Eduardo Habkost
2018-07-10 18:45 ` Fenghua Yu
2018-07-10 18:54 ` Dave Hansen
2018-07-10 19:47 ` Thomas Gleixner
2018-07-11 19:59 ` Dave Hansen
2018-07-12 20:00 ` Thomas Gleixner
2018-06-29 14:33 ` [PATCH v2 2/4] x86/split_lock: Align x86_capability to unsigned long to avoid split locked access Fenghua Yu
2018-06-29 16:04 ` Dave Hansen
2018-06-29 16:35 ` Thomas Gleixner
2018-06-29 19:03 ` Fenghua Yu
2018-06-29 20:08 ` Thomas Gleixner
2018-06-29 20:38 ` Fenghua Yu
2018-06-29 20:48 ` Dave Hansen
2018-06-29 21:10 ` Fenghua Yu
2018-06-29 21:44 ` Thomas Gleixner
2018-06-30 0:00 ` Fenghua Yu
2018-06-30 0:14 ` Fenghua Yu [this message]
2018-06-30 6:23 ` Thomas Gleixner
2018-07-02 12:18 ` Peter Zijlstra
2018-07-02 14:11 ` Fenghua Yu
2018-06-29 14:33 ` [PATCH v2 3/4] x86/split_lock: Handle #AC exception for split lock Fenghua Yu
2018-06-29 16:29 ` Dave Hansen
2018-06-29 16:33 ` Luck, Tony
2018-06-29 17:16 ` Fenghua Yu
2018-06-29 17:29 ` Dave Hansen
2018-06-29 17:39 ` Fenghua Yu
2018-06-29 17:47 ` Dave Hansen
2018-06-29 14:33 ` [PATCH v2 4/4] x86/split_lock: Disable #AC for split locked accesses Fenghua Yu
2018-06-29 16:31 ` Dave Hansen
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