[Acked/cmt] [PATCH Vivid/Wily SRU] x86/xen/p2m: hint at the last populated P2M entry

[Andy Whitcroft]

On Mon, Feb 08, 2016 at 11:13:09AM -0700, tim.gardner at canonical.com wrote:
> From: David Vrabel <david.vrabel at citrix.com>
> 
> BugLink: http://bugs.launchpad.net/bugs/1542941
> 
> With commit 633d6f17cd91ad5bf2370265946f716e42d388c6 (x86/xen: prepare
> p2m list for memory hotplug) the P2M may be sized to accomdate a much
> larger amount of memory than the domain currently has.
> 
> When saving a domain, the toolstack must scan all the P2M looking for
> populated pages.  This results in a performance regression due to the
> unnecessary scanning.
> 
> Instead of reporting (via shared_info) the maximum possible size of
> the P2M, hint at the last PFN which might be populated.  This hint is
> increased as new leaves are added to the P2M (in the expectation that
> they will be used for populated entries).
> 
> Signed-off-by: David Vrabel <david.vrabel at citrix.com>
> Cc: <stable at vger.kernel.org> # 4.0+
> (back ported from commit 98dd166ea3a3c3b57919e20d9b0d1237fcd0349d)

I think that is in a non-standard form.  Using the standard forms helps
things like the autotriager which read them.

> Signed-off-by: Tim Gardner <tim.gardner at canonical.com>
> 
> Conflicts:
> 	arch/x86/xen/p2m.c

I'd move those two up between the (backported ...) and Tim's SOB.

> ---
>  arch/x86/xen/p2m.c | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
> index 8b7f18e..912b123 100644
> --- a/arch/x86/xen/p2m.c
> +++ b/arch/x86/xen/p2m.c
> @@ -108,6 +108,15 @@ static unsigned long *p2m_identity;
>  static pte_t *p2m_missing_pte;
>  static pte_t *p2m_identity_pte;
>  
> +/*
> + * Hint at last populated PFN.
> + *
> + * Used to set HYPERVISOR_shared_info->arch.max_pfn so the toolstack
> + * can avoid scanning the whole P2M (which may be sized to account for
> + * hotplugged memory).
> + */
> +static unsigned long xen_p2m_last_pfn;
> +
>  static inline unsigned p2m_top_index(unsigned long pfn)
>  {
>  	BUG_ON(pfn >= MAX_P2M_PFN);
> @@ -262,7 +271,7 @@ void xen_setup_mfn_list_list(void)
>  
>  	HYPERVISOR_shared_info->arch.pfn_to_mfn_frame_list_list =
>  		virt_to_mfn(p2m_top_mfn);
> -	HYPERVISOR_shared_info->arch.max_pfn = xen_max_p2m_pfn;
> +	HYPERVISOR_shared_info->arch.max_pfn = xen_p2m_last_pfn;
>  }
>  
>  /* Set up p2m_top to point to the domain-builder provided p2m pages */
> @@ -394,6 +403,8 @@ void __init xen_vmalloc_p2m_tree(void)
>  	static struct vm_struct vm;
>  	unsigned long p2m_limit;
>  
> +	xen_p2m_last_pfn = xen_max_p2m_pfn;
> +
>  	p2m_limit = (phys_addr_t)P2M_LIMIT * 1024 * 1024 * 1024 / PAGE_SIZE;
>  	vm.flags = VM_ALLOC;
>  	vm.size = ALIGN(sizeof(unsigned long) * max(xen_max_p2m_pfn, p2m_limit),
> @@ -590,6 +601,12 @@ static bool alloc_p2m(unsigned long pfn)
>  			free_p2m_page(p2m);
>  	}
>  
> +	/* Expanded the p2m? */
> +	if (pfn > xen_p2m_last_pfn) {
> +		xen_p2m_last_pfn = pfn;
> +		HYPERVISOR_shared_info->arch.max_pfn = xen_p2m_last_pfn;
> +	}
> +
>  	return true;
>  }
>  

Acked-by: Andy Whitcroft <apw at canonical.com>

-apw