Re: mapping problems in xenpaging

[Olaf Hering <olaf@aepfle.de>]

On Thu, Sep 29, zhen shi wrote:

>  Hi,Olaf,
>  
>  When we analyze and test xenpaging,we found there are some problems between
> mapping and xenpaging.
>  1) When mapping firstly, then do xenpaging,and the code paths have resolved
> the problems.It's OK.
>  2) The problems exists if we do address mapping firstly then go to
> xenpaging,and our confusions are as followings:
>    a) If the domU's memory is directly mapped to dom0,such as the hypercall
> from pv driver,then it will build a related page-table in dom0,which will not
> change p2m-type.
>       and then do the xenpaging to page out the domU's memory pages whose gfn
> address have been already mapped to dom0;So it will cause some problems when
> dom0
>       accesses these pages.Because these pages are paged-out,and dom0 cannot
> tell the p2mt before access the pages.

I'm not entirely sure what you do. xenpaging runs in dom0 and is able to
map paged-out pages. It uses that to trigger a page-in, see
tools/xenpaging/pagein.c in xen-unstable.hg

>   b)The another situation is that if xen has mapped the domU's page, and get
> the mfn according to pfn_to_mfn.But then the page's p2mt is changed by others,
> so when xen
>     accesses the page ,it will cause problems such as BSOD or reboot.Because
> the operations of getting mfn and accessing the page are not
> atomic.and the situation exists
>     in many code paths .

Can you be more specific what you mean? Xen doesnt seem to have a
pfn_to_mfn function, only the tools have some helper macros of that name.


Olaf