Xen devel - May 2009 - About VMX VMExit handler

Hi,

In VM Exit handler, vmx_asm_vmexit_handler(),
[xen-3.3.0/xen/arch/x86/hvm/vmx/entry.S:line65], it first saves guest''s
RIP, RSP, RFLAGS from VMCS to stack by VMREAD, then call vmx_vmexit_handler, and
then recover guest''s RIP, RSP and RFLAGS.

1. The VMCS is designed to save these registers, why need to save them into
stack still?
2. What should I do, if I want to change the guest''s RIP in
vmx_vmexit_handler(), in order to let the guest load the new RIP on VM Entry? It
doesn''t work of changing the RIP in VMCS. Do I have to change the RIP
in the stack?

Thanks,
Wu

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>In VM Exit handler, vmx_asm_vmexit_handler(),
>[xen-3.3.0/xen/arch/x86/hvm/vmx/entry.S:line65], it first saves
>guest''s RIP, RSP, RFLAGS from VMCS to stack by VMREAD,
>then call vmx_vmexit_handler, and then recover guest''s RIP, RSP and RFLAGS.
>1. The VMCS is designed to save these registers,
>why need to save them into stack still?
These fields may be changed by common code, like shadow, you don''t want
to
Update VMCS directly in shadow code, or you''ll need arch specific
function pointers in shadow.
>2. What should I do, if I want to change the guest''s RIP
>in vmx_vmexit_handler(), in order to let the guest load the
>new RIP on VM Entry? It doesn''t work of changing the RIP
>in VMCS. Do I have to change the RIP in the stack?
Yes, we have __update_guest_eip for that purpose.
Thanks!
-Xin

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