Add support for Svade extension.

+static void p2m_set_permission(pte_t *e, p2m_type_t t) {

• panic("%s: hasn't been implemented yet\n", func);

• e->pte &= ~PTE_ACCESS_MASK;
• e->pte |= PTE_USER;
• /*

• * Two schemes to manage the A and D bits are defined:

• *   • The Svade extension: when a virtual page is accessed and the A bit

• *     is clear, or is written and the D bit is clear, a page-fault

• *     exception is raised.

• *   • When the Svade extension is not implemented, the following scheme

• *     applies.

• *     When a virtual page is accessed and the A bit is clear, the PTE is

• *     updated to set the A bit. When the virtual page is written and the

• *     D bit is clear, the PTE is updated to set the D bit. When G-stage

• *     address translation is in use and is not Bare, the G-stage virtual

• *     pages may be accessed or written by implicit accesses to VS-level

• *     memory management data structures, such as page tables.

• * Thereby to avoid a page-fault in case of Svade is available, it is

• * necesssary to set A and D bits.

• *

• * TODO: For now, it’s fine to simply set the A/D bits, since OpenSBI

• *       delegates page faults to a lower privilege mode and so OpenSBI

• *       isn't expect to handle page-faults occured in lower modes.

• *       By setting the A/D bits here, page faults that would otherwise

• *       be generated due to unset A/D bits will not occur in Xen.

• *

• *       Currently, Xen on RISC-V does not make use of the information

• *       that could be obtained from handling such page faults, which

• *       could otherwise be useful for several use cases such as demand

• *       paging, cache-flushing optimizations, memory access tracking,etc.

• *

• *       To support the more general case and the optimizations mentioned

• *       above, it would be better to stop setting the A/D bits here and

• *       instead handle page faults that occur due to unset A/D bits.

• */

• if ( riscv_isa_extension_available(NULL, RISCV_ISA_EXT_svade) )

•    e->pte |= PTE_ACCESSED | PTE_DIRTY;