VMCS Shadowing

VMCS Shadowing

[enrico canzonieri]

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Hi,
currently I am working on Xen because I am very interested in
virtualization technologies and I want to learn more about their
hypervisors internals. I would like to contribute at Xen community as
developer.


Looking at Intel software developer's manual I came across with VMCS
shadowing.
I would understand when it is used in Xen and with what advantages.
Is it really useful for nested virtualization?

Regards,
Enrico

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Re: VMCS Shadowing

[Tim Deegan]

At 21:17 +0200 on 26 May (1369603037), enrico canzonieri wrote:
> Hi,
> currently I am working on Xen because I am very interested in
> virtualization technologies and I want to learn more about their
> hypervisors internals. I would like to contribute at Xen community as
> developer.
> 
> 
> Looking at Intel software developer's manual I came across with VMCS
> shadowing.
> I would understand when it is used in Xen and with what advantages.
> Is it really useful for nested virtualization?

Yes, it has been used in Xen, for nested virtualization, since January:

commit 15e64e205e121c1eb18b6a9c1953c82443e4cd5a
Author: Dongxiao Xu <dongxiao.xu@intel.com>
Date:   Fri Jan 25 10:19:55 2013 +0100

    nested vmx: enable VMCS shadowing feature
    
    The current logic for handling the non-root VMREAD/VMWRITE is by
    VM-Exit and emulate, which may bring certain overhead.
    
    On new Intel platform, it introduces a new feature called VMCS
    shadowing, where non-root VMREAD/VMWRITE will not trigger VM-Exit,
    and the hardware will read/write the virtual VMCS instead.
    This is proved to have performance improvement with the feature.
    
    Signed-off-by: Dongxiao Xu <dongxiao.xu@intel.com>
    Acked-by: Jun Nakajima <jun.nakajima@intel.com>
    Acked-by Eddie Dong <eddie.dong@intel.com>
    Committed-by: Jan Beulich <jbeulich@suse.com>