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Re: [Xen-devel] XENFEAT_writable_pagetables vs VMASST_TYPE_writable_page

To: "Jacob Gorm Hansen" <jacobg@xxxxxxx>
Subject: Re: [Xen-devel] XENFEAT_writable_pagetables vs VMASST_TYPE_writable_pagetables
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Wed, 31 May 2006 16:00:00 +0100
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On 31 May 2006, at 15:22, Jacob Gorm Hansen wrote:

I am trying to figure out what the difference is between calling
vm-assist to enable writable page tables, and what the xen-feature
with the same name does. The former is enabled, while the latter
returns false. This has the effect that the various checks from
changeset 9243:f00e257d200c fail and pages are still pinned/unpinned,
but things still seem to work. This is with my own domain builder
using libxc.

So what is the story when combining VMASST_ and XENFEAT_?

It's a case of really bad naming: XENFEAT_writable_pagetables means that none of the pagetables need to be write-protected or pinned (presumably because you are permanently on shadow page tables). Whereas the vmassist simply means that you can attempt to directly write to your bottom-level PTEs, but page tables must generally be pinned and write-protected.

 -- Keir

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