From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Vrabel <david.vrabel@citrix.com>
Subject: Re: [RFC/PATCH] Improve speed of mapping guest memory
 into	Dom0
Date: Wed, 14 Nov 2012 16:39:40 +0000
Message-ID: <50A3C94C.2040806@citrix.com>
References: <50A37CC7.8050700@citrix.com> <50A397E1.7000602@citrix.com>
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To: David Vrabel <david.vrabel@citrix.com>
Cc: Mats Petersson <mats.petersson@citrix.com>, xen-devel@lists.xen.org, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
List-Id: xen-devel@lists.xenproject.org

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