linux-kernel - Re: [PATCH v2 2/4] x86/split_lock: Align x86_capability to unsigned long to avoid split locked access

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Date:   Fri, 29 Jun 2018 17:14:11 -0700
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Fenghua Yu <fenghua.yu@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...el.com>,
        Ingo Molnar <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Alan Cox <alan@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v2 2/4] x86/split_lock: Align x86_capability to unsigned
 long to avoid split locked access

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