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Re: [Xen-devel] Regarding page table management changes from Xen v1to Xe

To: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] Regarding page table management changes from Xen v1to Xen v2 (and v3)
From: Anthony Liguori <aliguori@xxxxxxxxxx>
Date: Wed, 26 Apr 2006 13:19:57 -0500
Cc: Ian Pratt <m+Ian.Pratt@xxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, Himanshu Raj <rhim@xxxxxxxxxxxxx>
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Keir Fraser wrote:

On 26 Apr 2006, at 19:15, Anthony Liguori wrote:

No, these singleton updates on the Linux demand fault path are issued
with the update_va_mapping hypercall. This is possible because the
virtual address that the pte maps is known, so its possible for Xen to
use a linear pagetable to do the update rather than creating an
on-demand mapping to the domain's pagetable page. This is an important
optimization on 32b where Xen address space is tight.

Just to make sure I understand, we do the linear mapping for performance (so we don't have to keep remapping) but not necessarily to save VA space since the linear mapping uses a fixed amount of space.

Is there any reason that we would strictly require a linear mapping (other than it being convenient)?

On 64b we could use the 1:1 mapping of all physmem that Xen has. Obviously that is infeasible on 32b. In fact the interface (passing a virtual address) pretty much means we have to use a linear mapping in both cases.

Couldn't we temporarily map the page table page? Not that we would actually want to do that. Just trying to confirm my understand.


Anthony Liguori

 -- Keir

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