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
index 2e20814d3ce3..ca62c3784f9a 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(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..24eac32b039d 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(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..fe681c695638 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(unsigned long);
+__u32 cpu_caps_set[NCAPINTS + NBUGINTS] __aligned(unsigned long);
void load_percpu_segment(int cpu)
{
diff --git a/include/linux/efi.h b/include/linux/efi.h
index 56add823f190..e1a3c17945b5 100644
--- a/include/linux/efi.h
+++ b/include/linux/efi.h
@@ -963,7 +963,7 @@ extern struct efi {
efi_reset_system_t *reset_system;
efi_set_virtual_address_map_t *set_virtual_address_map;
struct efi_memory_map memmap;
- unsigned long flags;
+ unsigned long flags __aligned(unsigned long);
} efi;
extern struct mm_struct efi_mm;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 5c91108846db..30b1f173d3ca 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -885,7 +885,7 @@ struct file {
void *f_security;
#endif
/* needed for tty driver, and maybe others */
- void *private_data;
+ void *private_data __aligned(unsigned long);
#ifdef CONFIG_EPOLL
/* Used by fs/eventpoll.c to link all the hooks to this file */
