hv: nested: Create a page pool for shadow EPT construction

Shadow EPT uses lots of pages to construct the shadow page table. To
utilize the memory more efficient, a page poll sept_page_pool is
introduced.

For simplicity, total platform RAM size is considered to calculate the
memory needed for shadow page tables. This is not an accurate upper
bound.  This can satisfy typical use-cases where there is not a lot
of overcommitment and sharing of memory between L2 VMs.

Memory of the pool is marked as reserved from E820 table in early stage.

Tracked-On: #5923
Signed-off-by: Sainath Grandhi <sainath.grandhi@intel.com>
Signed-off-by: Zide Chen <zide.chen@intel.com>
Signed-off-by: Shuo A Liu <shuo.a.liu@intel.com>
Reviewed-by: Jason Chen CJ <jason.cj.chen@intel.com>
Acked-by: Eddie Dong <eddie.dong@intel.com>

hypervisor/include/arch/x86/asm/guest/vept.h

@@ -7,6 +7,10 @@
 #define VEPT_H
 
 #ifdef CONFIG_NVMX_ENABLED
+void reserve_buffer_for_sept_pages(void);
+void init_vept(void);
 int32_t invept_vmexit_handler(struct acrn_vcpu *vcpu);
+#else
+static inline void reserve_buffer_for_sept_pages(void) {};
 #endif /* CONFIG_NVMX_ENABLED */
 #endif /* VEPT_H */