linux-kernel - Re: [Xen-devel] [PATCH 1/2] xen: prepare p2m list for memory hotplug

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Date:	Thu, 19 Mar 2015 17:22:01 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	Juergen Gross <jgross@...e.com>, <linux-kernel@...r.kernel.org>,
	<xen-devel@...ts.xensource.com>, <konrad.wilk@...cle.com>,
	<boris.ostrovsky@...cle.com>, <daniel.kiper@...cle.com>
Subject: Re: [Xen-devel] [PATCH 1/2] xen: prepare p2m list for memory hotplug

On 19/03/15 17:16, Juergen Gross wrote:
> On 03/19/2015 05:18 PM, David Vrabel wrote:
>> On 19/03/15 14:31, Juergen Gross wrote:
>>> Commit 054954eb051f35e74b75a566a96fe756015352c8 ("xen: switch to linear
>>> virtual mapped sparse p2m list") introduced a regression regarding to
>>> memory hotplug for a pv-domain: as the virtual space for the p2m list
>>> is allocated for the to be expected memory size of the domain only,
>>> hotplugged memory above that size will not be usable by the domain.
>>>
>>> Correct this by using a configurable size for the p2m list in case of
>>> memory hotplug enabled (default supported memory size is 512 GB for
>>> 64 bit domains and 4 GB for 32 bit domains).
>> [...]
>>> --- a/arch/x86/xen/p2m.c
>>> +++ b/arch/x86/xen/p2m.c
>>> @@ -91,6 +91,17 @@ EXPORT_SYMBOL_GPL(xen_p2m_size);
>>>   unsigned long xen_max_p2m_pfn __read_mostly;
>>>   EXPORT_SYMBOL_GPL(xen_max_p2m_pfn);
>>>
>>> +#ifdef CONFIG_XEN_BALLOON_MEMORY_HOTPLUG_LIMIT
>>> +#ifdef CONFIG_X86_32
>>> +BUILD_BUG_ON_MSG(CONFIG_XEN_BALLOON_MEMORY_HOTPLUG_LIMIT > 64)
>>> +#endif
>>> +#define P2M_LIMIT max(xen_max_p2m_pfn,                    \
>>> +    ((unsigned long)((u64)CONFIG_XEN_BALLOON_MEMORY_HOTPLUG_LIMIT
>>> *    \
>>> +    1024 * 1024 * 1024 / PAGE_SIZE)))
>>> +#else
>>> +#define P2M_LIMIT xen_max_p2m_pfn
>>> +#endif
>>
>> Can you arrange the #ifdef's to set xen_max_p2m_pfn to the right value
>> instead of introducing P2M_LIMIT?
> 
> Hmm, this would require additional checks in setup.c. What about:
> 
> #ifdef CONFIG_XEN_BALLOON_MEMORY_HOTPLUG_LIMIT
> #define P2M_LIMIT CONFIG_XEN_BALLOON_MEMORY_HOTPLUG_LIMIT
> #else
> #define P2M_LIMIT 0
> #endif
> 
> and do the max(...) calculation in xen_vmalloc_p2m_tree()?

Yes, this is fine.

David
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