Re: Grant Tables & mm.c

Re: Grant Tables & mm.c

[Koripella Srinivas]

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I was looking at the user space grant tables but i did not find any code calling the function
"remap_pfn_range" to map the kmalloc ed allocated memory into user space.
What could be the reason?? Or am i missing something

Thanks Mark.


----- Original Message ----
From: Mark Williamson <mark.williamson@cl.cam.ac.uk>
To: Koripella Srinivas <talkwithsrinivas@yahoo.co.in>
Cc: xen-devel@lists.xensource.com
Sent: Friday, 6 July, 2007 9:55:48 AM
Subject: Re: [Xen-devel] Grant Tables  & mm.c

> " Linux tends to zap userspace page mappings quite cheerfully
> unless you do something to stop it, could this be your problem?"
>
> What does this mean exactly??

Linux "zaps" mappings when it removes them from pagetables.  Really this 
corresponds to zeroing a pte corresponding to that mapping.

Unless you do something to stop the Linux mm system from treating the grant 
table mappins like "normal" RAM, it will eventually want to simply zero them.  
This will break for grant mappings, since it is not a permitted means of 
unmapping granted memory.

> Any way i can prevent it from happening?

It depends on what you're trying to do...

Have you looked at the userspace grantables driver in our Linux tree?  This 
provides a device for mapping granted memory safely into userspace, handling 
cleanup correctly.  Even if that's not directly useful, maybe you can find 
some code in there that'll help you.

Cheers,
Mark

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!







		
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Re: Grant Tables & mm.c

[Derek Murray]

On 7 Jul 2007, at 14:44, Koripella Srinivas wrote:

> I was looking at the user space grant tables but i did not find any  
> code calling the function
> "remap_pfn_range" to map the kmalloc ed allocated memory into user  
> space.
> What could be the reason?? Or am i missing something

The mapping takes place in drivers/xen/gntdev/gntdev.c, in the  
function gntdev_mmap(). In the normal case (no shadow page tables),  
the page is mapped into user space by calling  
HYPERVISOR_grant_table_op, with the GNTTABOP_map_grant_ref operation,  
and (amongst other things) a pointer to the page table entry to be  
rewritten. Therefore, it is not necessary to call remap_pfn_range().

I'm not sure what you mean by "the kmalloc ed allocated memory". The  
memory is allocated by another domain, and inserted in the grant  
table. In the kernel that carries out the mapping, kmalloc is used  
only for allocating kernel-private data structures.

Regards,

Derek Murray.

question regarding struct xc_dom_image

[tgh]

hi
  I try to read the function of "int xc_dom_boot_image(struct 
xc_dom_image *dom)",and I am confused about the two fields of struct 
xc_dom_image,p2m_guest and p2m_host,
 what are the meaning and functions of p2m_guest and p2m_host?

could you help me
Thanks in advance