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Re: [Xen-devel] [PATCH 7/8] docs: Document start_info changes in Xen 4.2.

On Mon, 2013-01-28 at 18:32 +0000, Konrad Rzeszutek Wilk wrote:
> The git commit 7a9d7646307b7c872b8dbd7546579acd3b54223d "x86/32-on-64:
> adjust Dom0 initial page table layout" fixes a bug in the reported
> value of pt_base versus what is stored in the %cr3 register. This
> documents this in the start of the world header note.
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> ---
>  xen/include/public/xen.h | 7 +++++++
>  1 file changed, 7 insertions(+)
> diff --git a/xen/include/public/xen.h b/xen/include/public/xen.h
> index c50bbfc..1685317 100644
> --- a/xen/include/public/xen.h
> +++ b/xen/include/public/xen.h
> @@ -705,6 +705,13 @@ typedef struct shared_info shared_info_t;
>   *  8. There is guaranteed to be at least 512kB padding after the final
>   *     bootstrap element. If necessary, the bootstrap virtual region is
>   *     extended by an extra 4MB to ensure this.
> + *
> + * Note: Prior to 7a9d7646307b7c872b8dbd7546579acd3b54223d ("x86/32-on-64:
> + * adjust Dom0 initial page table layout") the 3.e) contained two different
> + * values with a 64-bit hypervisor and a 32-bit initial domain kernel. The
> + * pt_base pointed to the L4 (setup by the hypervisor and not used by
> + * the guest) and the %cr3 pointed to the L3. This meant an difference of
> + * one page.
>   */

7a9d7646307b7c872b8dbd7546579acd3b54223d is not a Xen revision. I think
you meant 25833:bb85bbccb1c9. I assume this was a bug fix, in which case
I would say "Prior to Xen commit xxx:yyyy ("x86/32-on...") a bug caused
the pt_base (3.e above) to contain ..."

It would be useful to explain clearly the after case too, which I
suppose is that on 32-on-64 the pt_base now points to what the guest
kernel should consider to be the base? Would it be relevant to explain
how to detect this situation and what to do about it?

The difference of one page was just an implementation artefact, rather
than a meaningful semantic difference? Unless it is relevant to
detecting the situation I would be inclined not to do it.

It should also be made clearer that this affected only 32-on-64 dom0
kernels and not 32-on-64 domU or 64-bit kernels of any colour.


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