Re: [Xen-devel] [PATCH][HVM] Removing 1:1 mapping from qemu-dm

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 28 Jun 2006, at 16:15, Nakajima, Jun wrote:

> Since the write refcount in type_info can change at any time now 
> because
> of map/unmap by qemu-dm at runtime, I stopped using it to avoid a hack
> in mm.c. The extra code is a scaled-down version of write refcount
> dedicated for guest page table pages, and I think 4 bits would be
> sufficient in normal cases because normal guests don't establish that
> many translations (i.e. using different virtual addresses) against page
> table pages.
>
> Since the number cannot exceed the length of the shadow hash chains
> anyway, I can add an extra logic that detects overflow and scans the
> entire chains if detected. With this we can just have a 2-bit ref 
> count,
> 0 - no, 1 (most cases), and 2 - more than one.
I'd prefer the hack in mm.c I think. It's certainly much smaller, and 
it'd be okay with a comment. Is there any way for get/put_page_type to 
know if they've been called on a shadow page (as opposed to a guest 
page) and to avoid changing the type count based on that, rather than 
whether the page is local or foreign? That'd be a neater and less hacky 
thing to use for the decision imo.
 -- Keir


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