Xen devel - Apr 2007 - confused about the balloon code

hi
 I try to understand the code of balloon ,and got confused about the 
following parts of code:

static int decrease_reservation(unsigned long nr_pages)
{
         ....
        if (!PageHighMem(page)) {
            v = phys_to_virt(pfn << PAGE_SHIFT);
            scrub_pages(v, 1);
            ret = HYPERVISOR_update_va_mapping(
                (unsigned long)v, __pte_ma(0), 0);
            BUG_ON(ret);
        }
#ifdef CONFIG_XEN_SCRUB_PAGES
        else {
            v = kmap(page);
            scrub_pages(v, 1);
            kunmap(page);
        }
#endif
     ....
}

I am confused about the scrub_pages(), what is the meaning of 
scrub_pages, when should we use CONFIG_XEN_SCRUB_PAGES
could you help me
Thanks in advance



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On 28/4/07 02:13, "tgh" <tianguanhua@ncic.ac.cn> wrote:
> I am confused about the scrub_pages(), what is the meaning of
> scrub_pages, when should we use CONFIG_XEN_SCRUB_PAGES
> could you help me
It causes the guest to zero out pages that it gives back to Xen. It''s a
security option.

 -- Keir



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Thank you for your reply

and I have got other confusions in function of increase_reservation() as 
follows :
static int increase_reservation(unsigned long nr_pages)
{
....
page = balloon_first_page();
for (i = 0; i < nr_pages; i++) {
BUG_ON(page == NULL);
////**********************************
frame_list[i] = page_to_pfn(page);;
////*********** my confusion
////***********frame_list[i] is set by a pfn, while in the later part of 
this function and in decrease_reservation() ,frame_list[i] is set by 
mfn,is it right?
////*********** if it is , what is the trick behind it?
////************************************

page = balloon_next_page(page);
}

set_xen_guest_handle(reservation.extent_start, frame_list);
reservation.nr_extents = nr_pages;
////**********************************************
rc = HYPERVISOR_memory_op(
XENMEM_populate_physmap, &reservation);
////************************another confusion
////************************what is the function of 
XENMEM_populate_physmap hypercall?
////************************ is it to allocate the mfn for the guestOS 
and at that time ,the frame_list[i] is mfn not the pfn as above , or 
what is the function?
////**************************************************

....

for (i = 0; i < nr_pages; i++) {
page = balloon_retrieve();
BUG_ON(page == NULL);

pfn = page_to_pfn(page);
BUG_ON(!xen_feature(XENFEAT_auto_translated_physmap) &&
phys_to_machine_mapping_valid(pfn));

/* Update P->M and M->P tables. */
set_phys_to_machine(pfn, frame_list[i]);
xen_machphys_update(frame_list[i], pfn);

/* Link back into the page tables if not highmem. */
if (pfn < max_low_pfn) {
int ret;
////********************************************************
ret = HYPERVISOR_update_va_mapping(
(unsigned long)__va(pfn << PAGE_SHIFT),
pfn_pte_ma(frame_list[i], PAGE_KERNEL),
0);
////***********************another confustion
////***********************what is the function of this hypercall?
////*********************** I am totally confused about it
////********************************************************
BUG_ON(ret);
}

/* Relinquish the page back to the allocator. */
ClearPageReserved(page);
set_page_count(page, 1);
__free_page(page);
}
...
}

I am confused about it
could you help me
Thanks in advance










Keir Fraser 写道:
> On 28/4/07 02:13, "tgh" <tianguanhua@ncic.ac.cn> wrote:
>
>   
>> I am confused about the scrub_pages(), what is the meaning of
>> scrub_pages, when should we use CONFIG_XEN_SCRUB_PAGES
>> could you help me
>>     
>
> It causes the guest to zero out pages that it gives back to Xen.
It''s a
> security option.
>
>  -- Keir
>
>
>
>
>   

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Keir:

It makes sense from memory/page management stand-point,
I don''t understand any direct security link? May be I am taking it too
literally.

Please clarify.

-Kaushik 

-----Original Message-----
From: xen-devel-bounces@lists.xensource.com
[mailto:xen-devel-bounces@lists.xensource.com] On Behalf Of Keir Fraser
Sent: Saturday, April 28, 2007 1:17 AM
To: tgh; xen-devel@lists.xensource.com
Subject: Re: [Xen-devel] confused about the balloon code

On 28/4/07 02:13, "tgh" <tianguanhua@ncic.ac.cn> wrote:
> I am confused about the scrub_pages(), what is the meaning of
> scrub_pages, when should we use CONFIG_XEN_SCRUB_PAGES
> could you help me
It causes the guest to zero out pages that it gives back to Xen. It''s a
security option.

 -- Keir



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Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel

_______________________________________________
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On 28/4/07 19:04, "Kaushik Barde" <Kaushik_Barde@Phoenix.com>
wrote:
> It makes sense from memory/page management stand-point,
> I don''t understand any direct security link? May be I am taking it
too
> literally.
The only reason to scrub the pages is because you don''t want the domain
to
which they are next allocated to see their old contents.

 -- Keir
 



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