Re: Is the Guest OS in non-paging protected mode when VM first execute vmlaunch?

[Keir Fraser <keir.xen@gmail.com>]

On 21/05/2011 17:08, "henanwxr" <henanwxr@163.com> wrote:

> 1、Is the Guest OS in non-paging protected mode when VM first execute
> vmlaunch?
>    If it is so, why not set it in paging protected mode, could I do this?

I suppose, if you had reason to. Seems pointless though as you could set it
up more easily from within the guest.

> 2、I found guest os was set in paging and protected mode in function of
> construct_vmcs().

You are mixing up two things: the CR0 the guest *thinks* it is running with,
and the CR0 value it is *actually* running with. Actually we must always run
non-root mode with paging enabled, but we hide that from the guest by
running it on a hidden page table with 1:1 virt:phys mappings. So the
v->arch.hvm_vcpu.guest_cr[0] value in the code below is the cr0 value the
guest manages and sees. The v->arch.hvm_vcpu.hw_cr[0] is the value it
actually runs with. If you don't understand the difference between these
values, you'll get very confused!

 -- Keir

> The flow is:
> 
> //Xen-4.0/arch/x86/hvm/vmx/vmcs.c
> construct_vmcs()
> {
>   ………………..
>    //here set CR0.PE
>     v->arch.hvm_vcpu.guest_cr[0] = X86_CR0_PE | X86_CR0_ET;
>     hvm_update_guest_cr(v, 0);
> …………………
> }
> 
> 
> 
> 
> // Then it calls function of hvm_update_guest_cr() in
> /xen/include/asm-x86/hvm.h
>     static inline void hvm_update_guest_cr(struct vcpu *v, unsigned int cr)
> {
>     hvm_funcs.update_guest_cr(v, cr);
> }
> 
> 
> 
>     //hvm_funcs.update_guest_cr calls function of  vmx_updata_guest_cr()
>     //in /arch/x86/hvm/vmx/vmx.c
> static struct hvm_function_table __read_mostly vmx_function_table = {
>     .name                 = "VMX",
>     …………………………
>     .update_host_cr3      = vmx_update_host_cr3,
>     .update_guest_cr      = vmx_update_guest_cr,
>    ……………………..
> }
> 
> static void vmx_update_guest_cr(struct vcpu *v, unsigned int cr)
> {
>     vmx_vmcs_enter(v);
> 
>     switch ( cr )
>     {
>     case 0: {
>         int realmode;
>         unsigned long hw_cr0_mask = X86_CR0_NE;
>       
>  //here set CR0.PG and CR0.PE
>         if ( !vmx_unrestricted_guest(v) )
>          hw_cr0_mask |= X86_CR0_PG | X86_CR0_PE;
>           …………………………..
>           …………………………..
>          v->arch.hvm_vcpu.hw_cr[0] = v->arch.hvm_vcpu.guest_cr[0] |
> hw_cr0_mask;
>      //here write GUEST_CR0, is it in paging and protected mode ??
>      __vmwrite(GUEST_CR0, v->arch.hvm_vcpu.hw_cr[0]);
>      __vmwrite(CR0_READ_SHADOW, v->arch.hvm_vcpu.guest_cr[0]);
>       ………….      
> }
> 
> 
> --
> View this message in context:
> http://xen.1045712.n5.nabble.com/Is-the-Guest-OS-in-non-paging-protected-mode-
> when-VM-first-execute-vmlaunch-tp4415142p4415142.html
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> 
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