Re: [Xen-devel] [PATCH 4 of 4] xen/pagetables: Document pt_base inconsistency when running in COMPAT mode

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2012-08-21 at 21:08 +0100, Konrad Rzeszutek Wilk wrote:
> # HG changeset patch
> # User Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> # Date 1345579709 14400
> # Node ID b992f8e613a3401b9ddd140ce436c840d412beb7
> # Parent  74bedb086c5b72447262e087c0218b89f8bc9140
> xen/pagetables: Document pt_base inconsistency when running in COMPAT mode.
> 
> c/s 13257 added the COMPAT mode wherein a 64-bit hypervisor can
> run with a 32-bit initial domain. One of the things it added was
> that the pt_base is offset by two pages. This inconsistency
> is only present in the COMPAT mode so lets document it.
> 
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> 
> diff -r 74bedb086c5b -r b992f8e613a3 xen/include/public/xen.h
> --- a/xen/include/public/xen.h        Tue Aug 21 16:08:29 2012 -0400
> +++ b/xen/include/public/xen.h        Tue Aug 21 16:08:29 2012 -0400
> @@ -663,7 +663,7 @@ typedef struct shared_info shared_info_t
>   *      c. list of allocated page frames [mfn_list, nr_pages]
>   *         (unless relocated due to XEN_ELFNOTE_INIT_P2M)
>   *      d. start_info_t structure        [register ESI (x86)]
> - *      e. bootstrap page tables         [pt_base, CR3 (x86)]
> + *      e. bootstrap page tables         [pt_base, CR3 (x86)] *1
>   *      f. bootstrap stack               [register ESP (x86)]
>   *  4. Bootstrap elements are packed together, but each is 4kB-aligned.
>   *  5. The initial ram disk may be omitted.
> @@ -678,6 +678,9 @@ typedef struct shared_info shared_info_t
>   *
>   *  NOTE: The initial virtual region (3a -> 3f) are all mapped by the initial
>   *  pagetables [pt_base, CR3 (x86)].
> + *
> + *  *1: When booting under a 64-bit hypervisor with a 32-bit initial domain
> + *  it is offset by two pages (pt_base += PAGE_SIZE*2).

"it" here being the page which you would have to load into cr3?

What, if anything, ends up in the other two pages?

Ian.


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