Re: [PATCH 3/3] x86/xen: optimize get_phys_to_machine()

[Juergen Gross <jgross@suse.com>]

On 01/07/2015 03:47 PM, David Vrabel wrote:
> The page table walk is only needed to distinguish between identity and
> missing, both of which have INVALID_P2M_ENTRY.

As get_phys_to_machine is called by __pfn_to_mfn() only which already
checks for mfn == INVALID_P2M_ENTRY this optimization will have an
effect only in the early boot case with pfn >= xen_p2m_size.

I doubt this is necessary.


Juergen

>
> Signed-off-by: David Vrabel <david.vrabel@citrix.com>
> ---
>   arch/x86/xen/p2m.c |   30 ++++++++++++++++++------------
>   1 file changed, 18 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
> index edbc7a6..a848201 100644
> --- a/arch/x86/xen/p2m.c
> +++ b/arch/x86/xen/p2m.c
> @@ -405,8 +405,7 @@ void __init xen_vmalloc_p2m_tree(void)
>
>   unsigned long get_phys_to_machine(unsigned long pfn)
>   {
> -	pte_t *ptep;
> -	unsigned int level;
> +	unsigned long mfn;
>
>   	if (unlikely(pfn >= xen_p2m_size)) {
>   		if (pfn < xen_max_p2m_pfn)
> @@ -414,19 +413,26 @@ unsigned long get_phys_to_machine(unsigned long pfn)
>
>   		return IDENTITY_FRAME(pfn);
>   	}
> +	
> +	mfn = xen_p2m_addr[pfn];
>
> -	ptep = lookup_address((unsigned long)(xen_p2m_addr + pfn), &level);
> -	BUG_ON(!ptep || level != PG_LEVEL_4K);
> +	if (unlikely(mfn == INVALID_P2M_ENTRY)) {
> +		pte_t *ptep;
> +		unsigned int level;
>
> -	/*
> -	 * The INVALID_P2M_ENTRY is filled in both p2m_*identity
> -	 * and in p2m_*missing, so returning the INVALID_P2M_ENTRY
> -	 * would be wrong.
> -	 */
> -	if (pte_pfn(*ptep) == PFN_DOWN(__pa(p2m_identity)))
> -		return IDENTITY_FRAME(pfn);
> +		ptep = lookup_address((unsigned long)(xen_p2m_addr + pfn), &level);
> +		BUG_ON(!ptep || level != PG_LEVEL_4K);
> +
> +		/*
> +		 * The INVALID_P2M_ENTRY is filled in both p2m_*identity
> +		 * and in p2m_*missing, so returning the INVALID_P2M_ENTRY
> +		 * would be wrong.
> +		 */
> +		if (pte_pfn(*ptep) == PFN_DOWN(__pa(p2m_identity)))
> +			return IDENTITY_FRAME(pfn);
> +	}
>
> -	return xen_p2m_addr[pfn];
> +	return mfn;
>   }
>   EXPORT_SYMBOL_GPL(get_phys_to_machine);
>
>