[Xen-devel] Re: vmcs GUEST_CR0 unused?

[Anthony Liguori <aliguori@xxxxxxxxxxxxxxxxxx>]

Robert Phillips wrote:
> Why does the vmx code maintain hvm_vmx.cpu_cr0?
The implementation of hvm_funcs.get_guest_ctrl_reg() would be awkward 
since you would have make sure to load the vmcs for the VCPU you're 
interested in on the current PCPU before attempting to vmread(GUEST_CR0).
Regards,

Anthony Liguori

> I see code in vmx.c that keeps v->arch.hvm_vmx.cpu_cr0 up to date, and 
> each change is faithfully written to the vmcs using __vmwrite(GUEST_CR0, 
> ...)
> I also see that the CR0_GUEST_HOST_MASK is always all ones (~0UL), set 
> in construct_vmcs() and never modified.
> However according to section 2.6.6 of the VT specification the value in 
> GUEST_CR0 is irrelevant if CR0_GUEST_HOST_MASK is all ones.
> When the guest reads CR0, the mask will force it to see only the bits in 
> CR0_READ_SHADOW.
> When the guest modifies CR0, the mask will force a vmexit.
>
> So the vmcs value in GUEST_CR0 is never visible to the guest and never 
> really needed by the host.
> It looks to me like the code that maintains hvm_vmx.cpu_cr0 and 
> GUEST_CR0 is superfluous.
> The same argument applies to hvm_vmx.cup_cr4 and GUEST_CR4.
>
> Am I missing something?
>
> --
> --------------------------------------------------------------------
> Robert S. Phillips                          Virtual Iron Software
> rphillips@xxxxxxxxxxxxxxx 
> <mailto:rphillips@xxxxxxxxxxxxxxx>                Tower 1, Floor 2
> 978-849-1220                                 900 Chelmsford Street
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>
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