Xen devel - Jan 2007 - Informatio required on VMEXit settings and process.

Hi all,
Please help me in understanding who update the VMEXIT_REASON and
VMEXIT_QUALIFICATION fields in the VMCS field i.e. Xen or CPU and how when the
VMExit occurs.And how the VMexit occurs(porcess of vmexit).
Also explain this process in case of page fault if possible.
Give the code reference if possible.

Thanks and Regards,
Aditya Shevalkar.


		
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> -----Original Message-----
> From: xen-devel-bounces@lists.xensource.com 
> [mailto:xen-devel-bounces@lists.xensource.com] On Behalf Of 
> aditya shevalkar
> Sent: 30 January 2007 12:47
> To: xen devel
> Subject: [Xen-devel] Informatio required on VMEXit settings 
> and process.
> 
> Hi all,
> Please help me in understanding who update the VMEXIT_REASON 
> and VMEXIT_QUALIFICATION fields in the VMCS field i.e. Xen or 
> CPU and how when the VMExit occurs.And how the VMexit 
> occurs(porcess of vmexit).
The VMEXIT_REASON is set by the CPU when it sees a condition that
generates a vmexit (i.e. the guest is doing something that the
hypervisor/host has set to exit in "EXEC_CONTROLS"). The
VMEXIT_QUALIFICATION is likewise filled in with the relevant information
by the CPU (such as the address of a page-fault or the interrupt vector
on an interrupt exit). 

[I''m not an Intel expert, but seeing as the overall concept is almost
identical for AMD and Intel, the question can be answered in a way that
applies to both, with the slight difference that the AMD uses a VMCB and
different names for the fields. For example, exit_reason in Intel is
called exitcode in AMD''s world, and exit_qualification is called
exitinfo{1,2} and exitintinfo]
> Also explain this process in case of page fault if possible.
Processor sees a memory access. The processor checks the page-tables,
and if the access is not allowed, the processor gives a vmexit to the
hypervisor. The hypervisor checks the exit_reason to see that it''s a
page-fault, and determines if it''s a "hypervisor" related
page-fault
(such as a page-table update or a MMIO access to a virtual device) or a
"guest fault". For the former, the hypervisor will then perform the
relevant work to either update the page-table or to send MMIO operation
to QEMU. If it''s not a hypervisor fault, it is by process of
elimination
a guest-fault (for example a page-access that hits a page that is stored
in the swap or a write to a page that is shared with "copy-on-write"
in
a forked process). Guest faults are forwarded to the guest by
"injection". 

A more concrete question would probably give you a more precise answer -
there are MANY pages in the manual on the subject, so it''s pretty much
a
waste of everyones time to rewrite it here... 
> Give the code reference if possible.
.../xen/arch/x86/hvm/svm/svm.c for AMD code. 
.../xen/arch/x86/hvm/vmx/vmx.c for Intel code. 

--
Mats
> 
> Thanks and Regards,
> Aditya Shevalkar.
> 
> 
> 		
> __________________________________________________________
> Yahoo! India Answers: Share what you know. Learn something new
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> 
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