RE: [Xen-devel] VCPU Structure

["Nimgaonkar, Satyajeet" <SatyajeetNimgaonkar@xxxxxxxxxx>]

Hi Gianni,
               Thanks for your reply. I would certainly prefer to lock the 
pages instead. I checked the source code and in libxc/xc_domain.c, I see some 
code on locking the pages for a hypercall. I my case too, i need to lock memory 
pages assigned to a hypercall. Is that a right place I am looking at or can you 
please redirect me some code samples about locking memory pages.
               Thanks once again.

Regards,
Satyajeet
________________________________________
From: Gianni Tedesco [gianni.tedesco@xxxxxxxxxx]
Sent: Friday, November 12, 2010 12:00 PM
To: Nimgaonkar, Satyajeet
Cc: Kaushik Barde; Xen Devel
Subject: RE: [Xen-devel] VCPU Structure

On Fri, 2010-11-12 at 17:43 +0000, Nimgaonkar, Satyajeet wrote:
> Hi Kaushik,
>                  Here is what I wish to do.
>                  I have implemented a custom hypercall in xen. Now i
> need to access the memory pages that are being assigned to a
> particular process. I intend to do this from the hypercall handler
> itself. Once I have these memory pages, I will compute the hash
> (Memory Integrity block) of it and store it in the VCPU structure.
> This will help me save the state of the memory for a particular
> process. Later I can check if some modification have been done to the
> process by computing and matching the hash value with the one saved in
> the VCPU.
>                 It would be great if you could give me some insight on
> how to achieve this. Any sample code or examples on this would help a
> lot.
>                 Thanks a lot.

One way would be to use the MMU, you can mark those pages read-only in
the page table and then you will get a pagefault for any write to such a
page. In the fault handler you would mark the page as modified and
unprotect it by setting it read/write again.

That would save a lot of time doing expensive calculations when you
could just use the existing hardware acceleration designed for exactly
this purpose.

See Volume 3, Chapter 4 of the Intel architecture software developers
manual.

Gianni


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