Re: hvm trapping mechanism

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 23/12/2013 21:01, xennn wrote:
> Hi all,
>
> I would like to ask about hvm xen implementation. As far i understood the
> trapping-emulating flow is that: 
>
> 1. when hvm virtual machine is started vmcs is setup and a vm_exit handler
> is registered at: 
>
>  __vmwrite(HOST_RIP, (unsigned long)vmx_asm_vmexit_handler);
>
> 2. the hypercall page is defined by hypercall api and that hypercall page
> contains the vmexit calls.
> 3. when guest os issues syscall an hypercall is perfomed by the hypercall
> page and vmexit operation is peformed
> 4. the vmx_vmexit_handler(struct cpu_user_regs *regs) (vmx.c) is invoked to
> emulate the hypercall. 
>
> I would like to ask is this flow is right? Is there somethingimportant
> missing? 
> I would like to ask how the  vmx_vmexit_handler is invoked - may  be on
> hardware level or there is some other party at the flow?
>
> best regerds

Technically, the hypercall page contains 'vmcall' instructions.  There
is no such thing as a vmexit instruction.  A vmexit is the action of the
processor moving from non-root mode into root mode because of an action
requiring the intervention of the hypervisor.

vmx_asm_vmexit_handler is the handler for all vmexits, the vast majority
of which are not from hypercalls.  It includes may other things, such as
validity checks when the guest writes to control registers, or simply
that the real processor received an interrupt and that Xen should handle it.

~Andrew