Re: [Xen-devel] Confused about mapped pages "struct page" updates

["Waseem, Amna" <a.waseem@xxxxxxxxxxxxxxxxxxx>]

Thanks a lot
I understood it now 

________________________________________
From: Juergen Gross <jgross@xxxxxxxx>
Sent: Monday, November 6, 2017 11:24 AM
To: Waseem, Amna; Julien Grall
Cc: xen-devel; xen-devel@xxxxxxxxxxxxx
Subject: Re: [Xen-devel] Confused about mapped pages "struct page" updates

On 06/11/17 10:57, Waseem, Amna wrote:
> Hello All,
>
> I am a little confused about mapping mechanism in Xen for page from DomU to 
> Dom0.
>
> When Dom0 maps DomU page to its applied host_addr, Page table entries are 
> created by Xen hypervisor for mapping  applied host_addr vritual  address of 
> Dom0 to DomU physical page. The result is host_addr maps to DomU phsyical 
> page.
>
> Now in network backend driver, virt_to_page macro is called on this mapped 
> host_addr. How does Dom0 gets struct page for the mapped DomU page in its 
> domain? Is Xen also updates mem_map array of Dom0 to create struct page for 
> the mapped page? Or Dom0 creates struct page for all the physical memory 
> including provided to DomU during its creation ?
>
> Can anybody tell me how struct page for mapped pages from another domain gets 
> updated or created in DOm0?

Dom0 requests the mapping for a specific Dom0 physical address
(normally this is a page from the balloon driver, but in case no
ballooned page is available a kernel page is being allocated for
that purpose). So there always is a struct page available in Dom0.

host_addr above is part of Dom0 physical addresses. And the hypervisor
either modifies the Dom0 page table entry (in case of a PV Dom0 on
X86) or it just modifies the p2m mapping of the Dom0 physical address
(in case of ARM).


Juergen

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