What happens if I use get_user_pages()

What happens if I use get_user_pages()

[Arn]

Hi,

If I use the kernel function "get_user_pages()" within a guest kernel
module, will this ensure that the
guests pages are never swapped out by the host ? Will it lock these
pages in memory ?

(Has anyone tried this ? )

Thanks
Arn

Re: What happens if I use get_user_pages()

[Izik Eidus]

ציטוט Arn:
> Hi,
>
> If I use the kernel function "get_user_pages()" within a guest kernel
> module, will this ensure that the
> guests pages are never swapped out by the host ? Will it lock these
> pages in memory ?
>
> (Has anyone tried this ? )
>
> Thanks
> Arn
> --
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>   
running get_user_pages wont make the pages unswappable by the host (only 
by the guest)

Re: What happens if I use get_user_pages()

[Muli Ben-Yehuda]

On Tue, Jul 08, 2008 at 02:46:12AM -0400, Arn wrote:
> Hi,
> 
> If I use the kernel function "get_user_pages()" within a guest
> kernel module, will this ensure that the guests pages are never
> swapped out by the host ? Will it lock these pages in memory ?
> 
> (Has anyone tried this ? )

As Izik said, the answer is no. Is this the desirable outcome, or are
you trying to find a way for the guest to force the host to pin pages
in memory? If so, why?

Cheers,
Muli

Re: What happens if I use get_user_pages()

[Arn]

>> If I use the kernel function "get_user_pages()" within a guest
>> kernel module, will this ensure that the guests pages are never
>> swapped out by the host ? Will it lock these pages in memory ?
>> (Has anyone tried this ? )
>
> As Izik said, the answer is no. Is this the desirable outcome, or are
> you trying to find a way for the guest to force the host to pin pages
> in memory? If so, why?
>

Izik, Muli, thanks,

Actually I don't want the guests pages to be pinned at all. My aim is
to do a timing check from within guest (guest kernel) to see if we can
detect that the host has swapped out any of the guests pages. Without
the guest knowing. Its proving to be more difficult than I had
thought. Probably due to my lack of experience with kvm/linux
internals.

My idea is to get the guest kernel to read byte from each of its pages
(either all or a random set of pages) and do a timing check at each
read. A read from a page that has been swapped out by host should take
much longer than a read done directly from memory.

>From previous emails on this list it seems that if I map a guest's
page into its kernelspace then that page will never be swapped out
(due to shadow pagetables). So now I'm stuck, any advice will be
appreciated.

thanks
Arn

Re: What happens if I use get_user_pages()

[Izik Eidus]

ציטוט Arn:
>>> If I use the kernel function "get_user_pages()" within a guest
>>> kernel module, will this ensure that the guests pages are never
>>> swapped out by the host ? Will it lock these pages in memory ?
>>> (Has anyone tried this ? )
>>>       
>> As Izik said, the answer is no. Is this the desirable outcome, or are
>> you trying to find a way for the guest to force the host to pin pages
>> in memory? If so, why?
>>
>>     
>
> Izik, Muli, thanks,
>
> Actually I don't want the guests pages to be pinned at all. My aim is
> to do a timing check from within guest (guest kernel) to see if we can
> detect that the host has swapped out any of the guests pages. Without
> the guest knowing. Its proving to be more difficult than I had
> thought. Probably due to my lack of experience with kvm/linux
> internals.
>
> My idea is to get the guest kernel to read byte from each of its pages
> (either all or a random set of pages) and do a timing check at each
> read. A read from a page that has been swapped out by host should take
> much longer than a read done directly from memory.
>
> From previous emails on this list it seems that if I map a guest's
> page into its kernelspace then that page will never be swapped out
> (due to shadow pagetables). So now I'm stuck, any advice will be
> appreciated.
>   

pages that will be inside the shadow page tables wont be swappable, you 
have to use mmu notifiers patch in order to have reible swapping.

> thanks
> Arn
>