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Re: [PATCH v2 1/2] xen/domain: Annotate struct domain as page aligned
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 5 Mar 2025 10:23:14 +0100
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- Cc: Anthony PERARD <anthony.perard@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Julien Grall <julien@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Dario Faggioli <dfaggioli@xxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, George Dunlap <gwd@xxxxxxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Wed, 05 Mar 2025 09:23:20 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 04.03.2025 00:29, Andrew Cooper wrote:
> struct domain is always a page aligned allocation. Update it's type to
> reflect this, so we can safely reuse the lower bits in the pointer for
> auxiliary information.
>
> No functional change.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Anthony PERARD <anthony.perard@xxxxxxxxxx>
> CC: Michal Orzel <michal.orzel@xxxxxxx>
> CC: Jan Beulich <jbeulich@xxxxxxxx>
> CC: Julien Grall <julien@xxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> CC: Dario Faggioli <dfaggioli@xxxxxxxx>
> CC: Juergen Gross <jgross@xxxxxxxx>
> CC: George Dunlap <gwd@xxxxxxxxxxxxxx>
>
> v2:
> * New
>
> Interestingly this does cause two changes in the resulting binary, both caused
> by GCC electing to use a more complicated addressing mode to save one ADD
> instruction.
That's on x86, I suppose?
> --- a/xen/include/xen/sched.h
> +++ b/xen/include/xen/sched.h
> @@ -645,7 +645,7 @@ struct domain
> unsigned int num_llc_colors;
> const unsigned int *llc_colors;
> #endif
> -};
> +} __aligned(PAGE_SIZE);
>
> static inline struct page_list_head *page_to_list(
> struct domain *d, const struct page_info *pg)
I understand struct domain is where you need the annotation right away, but is
there a reason not to do the same for struct vcpu right away?
Jan
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