vmcs GUEST_CR0 unused?

vmcs GUEST_CR0 unused?

[Robert Phillips]

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Why does the vmx code maintain hvm_vmx.cpu_cr0?

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@virtualiron.com                Tower 1, Floor 2
978-849-1220                                 900 Chelmsford Street
                                                    Lowell, MA 01851

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Re: vmcs GUEST_CR0 unused?

[Anthony Liguori]

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@virtualiron.com 
> <mailto:rphillips@virtualiron.com>                Tower 1, Floor 2
> 978-849-1220                                 900 Chelmsford Street
>                                                     Lowell, MA 01851
> 
> 
> ------------------------------------------------------------------------
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel

Re: vmcs GUEST_CR0 unused?

[Keir Fraser]

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On 1/2/07 21:49, "Robert Phillips" <rsp.vi.xen@gmail.com> wrote:

> The same argument applies to hvm_vmx.cup_cr4 and GUEST_CR4.
> 
> Am I missing something?


Yes. GUEST_CR0 and GUEST_CR4 are the actual control-register values loaded
into the processor when running in VMX context. This can be different from
either the value the guest thinks it¹s running with, and also different from
the value loaded into the processor when running in root context (i.e., in
Xen).  They are cached in software structures because it can save a vmread
in some situations, which is worth around 50 cycles even on Core 2.

 -- Keir


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