Re: [head tinderbox] failure on sparc64/sun4v

On Wed, 3 Jun 2009, Marcel Moolenaar wrote:

>> Is there a reason not just to use __aligned(64) or the like on the first 
>> entry of the MD PCPU structure for sun4v to avoid future MI pcpu changes 
>> from causing similar discomfort for the MD pcpu parts?  Also, do we know 
>> why these alignment/sizing requirements exist for struct pcpu on sun4v but 
>> not other platforms?  If this is about packing pcpu structures into 
>> properly aligned cache lines, again __aligned() might be the right approach 
>> to take...
>
> Adding __aligned(xx) doesn't make it aligned. For example, malloc(3) only 
> aligns at 16-byte boundaries, so any user-space structure that has 
> __aligned(x>16) must manually make sure that this is actually the case by 
> over-allocating and then adjusting the pointer to an x>16 aligned address. 
> Likewise for the kernel, though it's easier in the kernel to get something 
> that's page-aligned... FYI,

I wan't sure if that was the problem that caused the alignment code in this 
case.  However, I agree that malloc(9)'s lack of alignment support is a 
problem, and one that should be pretty easy to resolve by simply putting a bit 
of over-allocation code in malloc(9), and adding a malloc_aligned(9) 
variation, or perhaps just an M_CACHEALIGN flag.

Robert N M Watson
Computer Laboratory
University of Cambridge