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

[Juergen Gross <jgross@xxxxxxxx>]

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@xxxxxxxxxx>
> ---
>   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);

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