KVM: VMX: disable SMEP feature when guest is in non-paging mode

SMEP is disabled if CPU is in non-paging mode in hardware.
However KVM always uses paging mode to emulate guest non-paging
mode with TDP. To emulate this behavior, SMEP needs to be manually
disabled when guest switches to non-paging mode.

We met an issue that, SMP Linux guest with recent kernel (enable
SMEP support, for example, 3.5.3) would crash with triple fault if
setting unrestricted_guest=0. This is because KVM uses an identity
mapping page table to emulate the non-paging mode, where the page
table is set with USER flag. If SMEP is still enabled in this case,
guest will meet unhandlable page fault and then crash.

Reviewed-by: Gleb Natapov <[email protected]>
Reviewed-by: Paolo Bonzini <[email protected]>
Signed-off-by: Dongxiao Xu <[email protected]>
Signed-off-by: Xiantao Zhang <[email protected]>
Signed-off-by: Marcelo Tosatti <[email protected]>

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