Re: [PATCH] x86: fix determination of bit count for struct domain allocations

[Keir Fraser <keir.xen@gmail.com>]

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> Jan Beulich <mailto:JBeulich@suse.com>
> 14 March 2014 15:23
> We can't just add in the hole shift value, as the hole may be at or
> above the 44-bit boundary. Instead we need to determine the total bit
> count until reaching 32 significant (not squashed out) bits in PFN
> representations.
>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>
> --- a/xen/arch/x86/domain.c
> +++ b/xen/arch/x86/domain.c
> @@ -180,6 +180,19 @@ void dump_pageframe_info(struct domain *
> spin_unlock(&d->page_alloc_lock);
> }
>
> +static unsigned int __init noinline _domain_struct_bits(void)
> +{
> + unsigned int bits = 32 + PAGE_SHIFT;
> + unsigned int sig = hweight32(~pfn_hole_mask);
> + unsigned int mask = pfn_hole_mask >> 32;
> +
> + for ( ; bits < BITS_PER_LONG && sig < 32; ++bits, mask >>= 1 )
> + if ( !(mask & 1) )
> + ++sig;

This logic took a few looks to be sure what it was doing, let alone 
correct. Without the patch comment I'd be even a bit more lost. I think 
the changeset comment, or similar, should be included as a code comment 
for this new function.

I'm sure given there is only one hole there is a simpler form of this 
based on checking whether bottom_shift > 44. But this is more general I 
suppose, so probably better despite being harder to understand.

Acked-by: Keir Fraser <keir@xen.org>
> + return bits;
> +}
> +
> struct domain *alloc_domain_struct(void)
> {
> struct domain *d;
> @@ -187,7 +200,10 @@ struct domain *alloc_domain_struct(void)
> * We pack the PDX of the domain structure into a 32-bit field within
> * the page_info structure. Hence the MEMF_bits() restriction.
> */
> - unsigned int bits = 32 + PAGE_SHIFT + pfn_pdx_hole_shift;
> + static unsigned int __read_mostly bits;
> +
> + if ( unlikely(!bits) )
> + bits = _domain_struct_bits();
>
> BUILD_BUG_ON(sizeof(*d) > PAGE_SIZE);
> d = alloc_xenheap_pages(0, MEMF_bits(bits));
>
>
>

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