Re: [Xen-devel] [RFC/PATCH] Improve speed of mapping guest memory into Dom0

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 14/11/12 13:08, David Vrabel wrote:
> On 14/11/12 11:13, Mats Petersson wrote:
> 
>> I have also found that the munmap() call used to unmap the guest memory
>> from Dom0 is about 35% slower in 3.7 kernel than in the 2.6 kernel (3.8M
>> cycles vs 2.8M cycles).
> 
> This performance reduction only occurs with 32-bit guests is the Xen
> then traps-and-emulates both halves of the PTE write.
> 
>> I think this could be made quicker by using a
>> direct write of zero rather than the compare exchange operation that is
>> currently used [which traps into Xen, performs the compare & exchange] -
> 
> This is something I noticed but never got around to producing a patch.
> How about this (uncomplied!) patch?
> 
> -- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1146,8 +1146,16 @@ again:
>                                    page->index > details->last_index))
>                                       continue;
>                       }
> -                     ptent = ptep_get_and_clear_full(mm, addr, pte,
> -                                                     tlb->fullmm);
> +                     /*
> +                      * No need for the expensive atomic get and
> +                      * clear for anonymous mappings as the dirty
> +                      * and young bits are not used.
> +                      */
> +                     if (PageAnon(page))

The mapping might not be backed by pages (e.g., foreign mappings) so:

if (!page || PageAnon(page))

> +                             pte_clear(mm, addr, pte);
> +                     else
> +                             ptent = ptep_get_and_clear_full(mm, addr, pte,
> +                                                             tlb->fullmm);
>                       tlb_remove_tlb_entry(tlb, pte, addr);
>                       if (unlikely(!page))
>                               continue;

David

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel