Re: raid6 badness
From: H. Peter Anvin (hpa_at_zytor.com)
Date: 01/31/04
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Date: Fri, 30 Jan 2004 21:50:23 -0800 To: Andi Kleen <ak@suse.de>
Andi Kleen wrote:
> "H. Peter Anvin" <hpa@zytor.com> writes:
>
>>I don't know what would cause the stack to be misaligned, however.
>
> x86-64 kernel doesn't guarantee the stack to be 16 byte aligned
> (although it usually is). If you need 16 byte alignment you have
> to align yourself.
>
OK, that's unfortunate... per our discussion I really think this is a
bug, since the compiler still does 16-byte alignment, and thus we're
taking the cost without the benefit.
I'll send in the attached patch for now, but at some point I'd like to
fix this. Unfortunately I still don't have an x86-64 machine that I can
actually compile and install kernels on; I only have access to an x86-64
userspace, so I'm a bit limited in what I can test.
Michael: Perhaps you could apply this patch and test it out for me?
-hpa
===================================================================
RCS file: /home/hpa/kernel/bkcvs/linux-2.5/drivers/md/raid6x86.h,v
retrieving revision 1.3
diff -u -r1.3 raid6x86.h
--- linux-2.5/drivers/md/raid6x86.h 22 Jan 2004 16:15:09 -0000 1.3
+++ linux-2.5/drivers/md/raid6x86.h 31 Jan 2004 05:41:49 -0000
@@ -32,18 +32,20 @@
/* N.B.: For SSE we only save %xmm0-%xmm7 even for x86-64, since
the code doesn't know about the additional x86-64 registers */
typedef struct {
- unsigned int sarea[8*4];
- unsigned int cr0;
+ unsigned int sarea[8*4+2];
+ unsigned long cr0;
} raid6_sse_save_t __attribute__((aligned(16)));
/* This is for x86-64-specific code which uses all 16 XMM registers */
typedef struct {
- unsigned int sarea[16*4];
+ unsigned int sarea[16*4+2];
unsigned long cr0;
} raid6_sse16_save_t __attribute__((aligned(16)));
-/* On x86-64 the stack is 16-byte aligned */
-#define SAREA(x) (x->sarea)
+/* On x86-64 the stack *SHOULD* be 16-byte aligned, but currently this
+ is buggy in the kernel and it's only 8-byte aligned in places, so
+ we need to do this anyway. Sigh. */
+#define SAREA(x) ((unsigned int *)((((unsigned long)&(x)->sarea)+15) & ~15))
#else /* __i386__ */
@@ -60,6 +62,7 @@
unsigned long cr0;
} raid6_sse_save_t;
+/* Find the 16-byte aligned save area */
#define SAREA(x) ((unsigned int *)((((unsigned long)&(x)->sarea)+15) & ~15))
#endif
-
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