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Re: [Xen-devel] [PATCH v3 12/20] xen/balloon: Don't rely on the page granularity is the same for Xen and Linux

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  Re: [Xen-devel] [PATCH v3 12/20] xen/balloon: Don't rely on the page  granularity is the same for Xen and Linux David Vrabel <david.vrabel@citrix.com> - 2015-08-20 12:10 +0200
    Re: [Xen-devel] [PATCH v3 12/20] xen/balloon: Don't rely on the page  granularity is the same for Xen and Linux Julien Grall <julien.grall@citrix.com> - 2015-08-28 17:20 +0200

#1210376 — Re: [Xen-devel] [PATCH v3 12/20] xen/balloon: Don't rely on the page granularity is the same for Xen and Linux

On 07/08/15 17:46, Julien Grall wrote:
> For ARM64 guests, Linux is able to support either 64K or 4K page
> granularity. Although, the hypercall interface is always based on 4K
> page granularity.
> 
> With 64K page granularity, a single page will be spread over multiple
> Xen frame.
> 
> To avoid splitting the page into 4K frame, take advantage of the
> extent_order field to directly allocate/free chunk of the Linux page
> size.
> 
> Note that PVMMU is only used for PV guest (which is x86) and the page
> granularity is always 4KB. Some BUILD_BUG_ON has been added to ensure
> that because the code has not been modified.
[...]
>  #ifdef CONFIG_XEN_HAVE_PVMMU
> +		/* We don't support PV MMU when Linux and Xen is using
> +		 * different page granularity.
> +		 */
> +		BUILD_BUG_ON(XEN_PAGE_SIZE != PAGE_SIZE);

You don't need this BUILD_BUG_ON() twice.

Otherwise,

Reviewed-by: David Vrabel <david.vrabel@citrix.com>

David
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#1215403

On 20/08/15 10:59, David Vrabel wrote:
> On 07/08/15 17:46, Julien Grall wrote:
>> For ARM64 guests, Linux is able to support either 64K or 4K page
>> granularity. Although, the hypercall interface is always based on 4K
>> page granularity.
>>
>> With 64K page granularity, a single page will be spread over multiple
>> Xen frame.
>>
>> To avoid splitting the page into 4K frame, take advantage of the
>> extent_order field to directly allocate/free chunk of the Linux page
>> size.
>>
>> Note that PVMMU is only used for PV guest (which is x86) and the page
>> granularity is always 4KB. Some BUILD_BUG_ON has been added to ensure
>> that because the code has not been modified.
> [...]
>>  #ifdef CONFIG_XEN_HAVE_PVMMU
>> +		/* We don't support PV MMU when Linux and Xen is using
>> +		 * different page granularity.
>> +		 */
>> +		BUILD_BUG_ON(XEN_PAGE_SIZE != PAGE_SIZE);
> 
> You don't need this BUILD_BUG_ON() twice.

I put twice the BUILD_BUG_ON so if we ever decide to drop one of the
#ifdef CONFIG_XEN_HAVE_PVMMU, the check will still be present.

So I'd like to keep it.

> Otherwise,
> 
> Reviewed-by: David Vrabel <david.vrabel@citrix.com>

Based on the discussion with Stefano I rework a bit the balloon code to
re-use lru field (see [1]), so I won't retain your reviewed-by on the
next series.

Regards,

[1] http://lists.xen.org/archives/html/xen-devel/2015-08/msg00781.html

-- 
Julien Grall
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