Allowing several mappings of a frame

Allowing several mappings of a frame

[Mohammad Hedayati]

Xen currently allows granting a single frame to several domains,
several times (to map or copy). Though the feature itself seems
appealing, I wonder if there's a real use case in any ports or PV
aware applications (i.e. a page is mapped or copied by more than 2
domains). Is there?

Except the condition outlined above, PGC_count_mask part of a
page->count_info can be more than 1?

Thanks,
Mohammad

Re: Allowing several mappings of a frame

[Keir Fraser]

On 27/10/2011 22:16, "Mohammad Hedayati" <hedayati.mo@gmail.com> wrote:

> Xen currently allows granting a single frame to several domains,
> several times (to map or copy). Though the feature itself seems
> appealing, I wonder if there's a real use case in any ports or PV
> aware applications (i.e. a page is mapped or copied by more than 2
> domains). Is there?

It wouldn't typically happen.

> Except the condition outlined above, PGC_count_mask part of a
> page->count_info can be more than 1?

Yes. Go trace some count_info values in a running system. Most will be >1.

 -- Keir

> Thanks,
> Mohammad
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel

Re: Allowing several mappings of a frame

[Mohammad Hedayati]

On Fri, Oct 28, 2011 at 1:25 AM, Keir Fraser <keir.xen@gmail.com> wrote:
> On 27/10/2011 22:16, "Mohammad Hedayati" <hedayati.mo@gmail.com> wrote:
>
>> Xen currently allows granting a single frame to several domains,
>> several times (to map or copy). Though the feature itself seems
>> appealing, I wonder if there's a real use case in any ports or PV
>> aware applications (i.e. a page is mapped or copied by more than 2
>> domains). Is there?
>
> It wouldn't typically happen.
>
>> Except the condition outlined above, PGC_count_mask part of a
>> page->count_info can be more than 1?
>
> Yes. Go trace some count_info values in a running system. Most will be >1.
>

I have traced the faulting pages in do_page_fault (where else?). The
pages with more than one reference (PGC_count_mask & page->count_info)
were flagged as PGT_lx_page_table or PGT_seg_desc_page (are the
mappers dom_xen, dom_cow or dom_io?). Is there more than one reference
for an ordinary pages?

>  -- Keir
>
>> Thanks,
>> Mohammad
>>
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@lists.xensource.com
>> http://lists.xensource.com/xen-devel
>
>
>

How can I know, in addition to the owner, who has mapped a page?

What does (PGT_count_mask &  page->u.inuse.type_info) show? Why it is
not the same as (PGC_count_mask & page->count_info)?

Is there a documentation other than the comments in the code?

Thanks,
Mohammad