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Re: [Xen-devel] Hypercall from HVM guest

To: xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-devel] Hypercall from HVM guest
From: "Emre Can Sezer" <ecsezer@xxxxxxxx>
Date: Thu, 25 Sep 2008 16:51:54 -0400 (EDT)
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> Emre Can Sezer wrote:
>> Hi,
>> I'm trying to communicate some information from an HVM guest to Xen.
Passing a pointer to a buffer would suffice and the communication will
>> single sided.  What is the best way to do this?
>> I've looked into making a hypercall from an HVM guest and came across this
>> post on this mailing list by Steve Ofsthun:
>> http://lists.xensource.com/archives/html/xen-devel/2006-04/msg00526.html
> This code is quite old and much has changed since then.  You should
probably use the pv on hvm driver code as an example these days
> (xen/unmodified_drivers/linux-2.6/platform-pci).
> What hypercalls are you trying to use?  Note that HVM guests are
restricted to a subset of the normal PV guest hypercalls.
> Steve

I'm trying to pass some information about the guest kernel to the VMM and
thought that passing a single pointer to a buffer would suffice.  I plan
on introducing a new hypercall for my purposes and update vmx.c in Xen. If
there is a better way of doing it please let me know.

I put together some code, mainly from platform-pci.c and your previous
posts and I was able to capture the hypercall from Xen.  I've attached the
code below.  My problem was the xen related header files in the new linux
distributions from kernel.org.  So instead of including them, I copied the
relevant information to the source.  I've attached the code below.

I wonder if this is a bug or if 64-bit systems aren't supported or if they
don't support xen anymore.  The __HYPERCALL_ definitions for example are
lacking what you have in xen.h in linux-2.6.18-xen.  Although they've come
up with 2.6.26-5, the changelogs don't mention anything about xen header

It would be great if there was a skeleton code that people could use. 
Perhaps someone would like to take this code and pretty it up? :)



>> However, I'm unable to get this code to work.  I'm running a 64-bit system
>> with the linux-2.6.26 kernel downloaded from kernel.org on top of
xen-3.2.2.  When I compile, I get compilation errors resulting from
hypercall.h.  I believe this code is intended for 32 bit guests only as
the structure definitions are different fot the pte_t type.
>> The error was:
>> In file included from include/asm/xen/hypervisor.h:53,
>>                  from /home/ecsezer/kernels/vmcall/./features.c:11:
>> include/asm/xen/hypercall.h: In function
'HYPERVISOR_update_va_mapping': include/asm/xen/hypercall.h:230: error:
'pte_t' has no member named 'pte_low'
>> I tried to modify this code to get rid of compilation errors but a
final warning still persists and loading the module causes a seg fault.
 The warning I get is:
>>   CC [M]  /home/ecsezer/kernels/hyper/./hypercall.o
>> {standard input}: Assembler messages:
>> {standard input}:149: Warning: indirect call without `*'
>> So I tried to compile with mercurial repository version
linux-2.6.18-xen and that compiles fine.  However, due to being a
different version, I can't use the resulting module.
>> I would appreciate any insight as to how to proceed.  What could be the
problem?  Perhaps there is an easier way of passing the information I
>> without a full blown hypercall interface?
>> Thanks,
>> John
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@xxxxxxxxxxxxxxxxxxx
>> http://lists.xensource.com/xen-devel

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