Re: [Xen-devel] [PATCH v3] x86/vmx: Add force-ept command line option

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Thu, Apr 17, 2014 at 8:16 PM, Aravindh Puthiyaparambil
<aravindp@xxxxxxxxx> wrote:
> Add a "force-ept" command line option to allow EPT to be enabled when
> VMX feature VM_ENTRY_LOAD_GUEST_PAT is not present.
>
> Due to CVE-2013-2212, this feature is required by default as a
> prerequisite for using EPT. If you are not using PCI Passthrough, or
> trust the guest administrator who would be using passthrough, then the
> requirement can be relaxed. This option is useful for nested
> virtualisation cases where the outer hypervisor does not expose the
> VMX feature VM_ENTRY_LOAD_GUEST_PAT to Xen.
>
> Signed-off-by: Aravindh Puthiyaparambil <aravindp@xxxxxxxxx>
> Cc: Jun Nakajima <jun.nakajima@xxxxxxxxx>
> Cc: Eddie Dong <eddie.dong@xxxxxxxxx>
> Cc: Kevin Tian <kevin.tian@xxxxxxxxx>
>
> ---
> Changes from version 2:
> 1. Update commit and documentation description.
> 2. Rename command line option to "force-ept"
>
> Changes from version 1:
> 1. Fix and update documentation with suggestion from Andrew Cooper.
> 2. Remove redundant assignment.
> ---
>  docs/misc/xen-command-line.markdown | 16 ++++++++++++++++
>  xen/arch/x86/hvm/vmx/vmx.c          |  5 ++++-
>  2 files changed, 20 insertions(+), 1 deletion(-)
>
> diff --git a/docs/misc/xen-command-line.markdown 
> b/docs/misc/xen-command-line.markdown
> index 87de2dc..6587069 100644
> --- a/docs/misc/xen-command-line.markdown
> +++ b/docs/misc/xen-command-line.markdown
> @@ -545,6 +545,22 @@ versa.  For example to change dom0 without changing 
> domU, use
>
>  Specify the font size when using the VESA console driver.
>
> +### force-ept (Intel)
> +> `= <boolean>`
> +
> +> Default: `false`
> +
> +Allow EPT to be enabled when VMX feature VM\_ENTRY\_LOAD\_GUEST\_PAT is not
> +present.
> +
> +*Warning:*
> +Due to CVE-2013-2212, VMX feature VM\_ENTRY\_LOAD\_GUEST\_PAT is by default
> +required as a prerequisite for using EPT.  If you are not using PCI 
> Passthrough,
> +or trust the guest administrator who would be using passthrough, then the
> +requirement can be relaxed.  This option is useful for nested virtualisation
> +cases where the outer hypervisor does not expose the VMX feature
> +VM\_ENTRY\_LOAD\_GUEST\_PAT to Xen.

Perhaps something like the following? "This option is particularly
useful for nested virtualization, to allow the L1 hypervisor to use
EPT even if the L0 hypervisor does not provide
VM\_ENTRY\_LOAD\_GUEST\_PAT."

I'm not sure "force-ept" is quite descriptive enough; it seems like it
would be better to mention the specific thing you're ignoring.
"ept-unsafe-pat" perhaps?

 -George

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