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Re: [Xen-devel] HYPERVISOR_update_va_mapping in mm/memory.c

To: Jacob Gorm Hansen <jacob@xxxxxxxx>
Subject: Re: [Xen-devel] HYPERVISOR_update_va_mapping in mm/memory.c
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Thu, 05 Feb 2004 17:43:18 +0000
Cc: Xen list <xen-devel@xxxxxxxxxxxxxxxxxxxxx>
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> hi,
> 
> I am trying to guess the purpose of the calls to
> HYPERVISOR_update_va_mapping in mm/memory.c, or rather why this code
> (which apparently handles user space demand paging) uses a special
> hypercall rather than the usual pte-update calls.
> 
> Could anyone briefly explain to me what this hypercall does, and why it
> needs to be special?
> 
> thanks,
> Jacob

It's faster to execute than doing the same thing via the usual update
hypercall, because:
 1. The code executes one special case so it is shorter and
 straight-line.
 2. Xen modifies the specified PTE directly using the guest-provided
 mapping of that PTE. This is faster than creating a temporary mapping
 of that page in Xen's address space.

Short answer: it's just a performance optimisation.

 -- Keir


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