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[PATCH v2 1/2] xen/domain: Annotate struct domain as page aligned

Andrew Cooper posted 2 patches 5 days, 22 hours ago
There is a newer version of this series
[PATCH v2 1/2] xen/domain: Annotate struct domain as page aligned
Posted by Andrew Cooper 5 days, 22 hours ago 
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@citrix.com>
---
CC: Anthony PERARD <anthony.perard@vates.tech>
CC: Michal Orzel <michal.orzel@amd.com>
CC: Jan Beulich <jbeulich@suse.com>
CC: Julien Grall <julien@xen.org>
CC: Roger Pau Monné <roger.pau@citrix.com>
CC: Stefano Stabellini <sstabellini@kernel.org>
CC: Dario Faggioli <dfaggioli@suse.com>
CC: Juergen Gross <jgross@suse.com>
CC: George Dunlap <gwd@xenproject.org>

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.
---
 xen/include/xen/sched.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
index 037c83fda219..8412b45178ca 100644
--- 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)
-- 
2.39.5
Re: [PATCH v2 1/2] xen/domain: Annotate struct domain as page aligned
Posted by Jan Beulich 4 days, 12 hours ago 
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@citrix.com>
> ---
> CC: Anthony PERARD <anthony.perard@vates.tech>
> CC: Michal Orzel <michal.orzel@amd.com>
> CC: Jan Beulich <jbeulich@suse.com>
> CC: Julien Grall <julien@xen.org>
> CC: Roger Pau Monné <roger.pau@citrix.com>
> CC: Stefano Stabellini <sstabellini@kernel.org>
> CC: Dario Faggioli <dfaggioli@suse.com>
> CC: Juergen Gross <jgross@suse.com>
> CC: George Dunlap <gwd@xenproject.org>
> 
> 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
Re: [PATCH v2 1/2] xen/domain: Annotate struct domain as page aligned
Posted by Andrew Cooper 3 days ago 
On 05/03/2025 9:23 am, Jan Beulich wrote:
> 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@citrix.com>
>> ---
>> CC: Anthony PERARD <anthony.perard@vates.tech>
>> CC: Michal Orzel <michal.orzel@amd.com>
>> CC: Jan Beulich <jbeulich@suse.com>
>> CC: Julien Grall <julien@xen.org>
>> CC: Roger Pau Monné <roger.pau@citrix.com>
>> CC: Stefano Stabellini <sstabellini@kernel.org>
>> CC: Dario Faggioli <dfaggioli@suse.com>
>> CC: Juergen Gross <jgross@suse.com>
>> CC: George Dunlap <gwd@xenproject.org>
>>
>> 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?

Yes.

>
>> --- 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?

struct vcpu is more complicated.  It's multi-page on ARM, and I have a
strong suspicion that alignment is going to have to change away from
PAGE_SIZE for architectures wanting to use larger page sizes.

e.g. with 64k pagetables, I expect these will still want 4k alignment,
and therefore they'll all want changing. i.e. I'm probably creating less
work for someone in the future by not annotating struct vcpu. ~Andrew
Re: [PATCH v2 1/2] xen/domain: Annotate struct domain as page aligned
Posted by Jan Beulich 2 days, 14 hours ago 
On 06.03.2025 22:29, Andrew Cooper wrote:
> On 05/03/2025 9:23 am, Jan Beulich wrote:
>> On 04.03.2025 00:29, Andrew Cooper wrote:
>>> --- 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?
> 
> struct vcpu is more complicated.  It's multi-page on ARM, and I have a
> strong suspicion that alignment is going to have to change away from
> PAGE_SIZE for architectures wanting to use larger page sizes.
> 
> e.g. with 64k pagetables, I expect these will still want 4k alignment,
> and therefore they'll all want changing. i.e. I'm probably creating less
> work for someone in the future by not annotating struct vcpu. ~Andrew

Wouldn't the same hold for struct domain?

Jan
Re: [PATCH v2 1/2] xen/domain: Annotate struct domain as page aligned
Posted by Stefano Stabellini 4 days, 22 hours ago 
On Mon, 3 Mar 2025, 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@citrix.com>

Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>


> ---
> CC: Anthony PERARD <anthony.perard@vates.tech>
> CC: Michal Orzel <michal.orzel@amd.com>
> CC: Jan Beulich <jbeulich@suse.com>
> CC: Julien Grall <julien@xen.org>
> CC: Roger Pau Monné <roger.pau@citrix.com>
> CC: Stefano Stabellini <sstabellini@kernel.org>
> CC: Dario Faggioli <dfaggioli@suse.com>
> CC: Juergen Gross <jgross@suse.com>
> CC: George Dunlap <gwd@xenproject.org>
> 
> 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.
> ---
>  xen/include/xen/sched.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
> index 037c83fda219..8412b45178ca 100644
> --- 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)
> -- 
> 2.39.5
>

Patchew 2.1-devel-b67e4c2

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