riscv64: explicit Bare-mode satp switch (item 4.3.5a)

Punch list §25 item 4.3.5a complete.

Firmware (EDK2 RISC-V) leaves Sv57 paging active at kernel entry with an
identity mapping that has at least one confirmed hole (PLIC threshold
register). Verified live before acting: satp.MODE/PPN and __kernel_start's
address confirmed identity mapping; switched to Bare mode (csrw satp, x0 +
sfence.vma) in arch_early_init(), ahead of pmm_init()/vmm_init().
Three-arch acceptance boot clean, zero exceptions; riscv64's PLIC_THRESHOLD
write now survives (PLIC driver code itself lands separately with 4.3.5b).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

logs/20260808-101931/riscv64/qemu-riscv64-20260808-101931.log

@@ -0,0 +1,127 @@
+
+OpenSBI v1.8
+   ____                    _____ ____ _____
+  / __ \                  / ____|  _ \_   _|
+ | |  | |_ __   ___ _ __ | (___ | |_) || |
+ | |  | | '_ \ / _ \ '_ \ \___ \|  _ < | |
+ | |__| | |_) |  __/ | | |____) | |_) || |_
+  \____/| .__/ \___|_| |_|_____/|____/_____|
+        | |
+        |_|
+
+Platform Name               : riscv-virtio,qemu
+Platform Features           : medeleg
+Platform HART Count         : 1
+Platform HART Protection    : pmp
+Platform IPI Device         : aclint-mswi
+Platform Timer Device       : aclint-mtimer @ 10000000Hz
+Platform Console Device     : uart8250
+Platform HSM Device         : ---
+Platform PMU Device         : ---
+Platform Reboot Device      : syscon-reboot
+Platform Shutdown Device    : syscon-poweroff
+Platform Suspend Device     : ---
+Platform CPPC Device        : ---
+Firmware Base               : 0x80000000
+Firmware Size               : 321 KB
+Firmware RW Offset          : 0x40000
+Firmware RW Size            : 65 KB
+Firmware Heap Offset        : 0x47000
+Firmware Heap Size          : 37 KB (total), 0 KB (reserved), 12 KB (used), 23 KB (free)
+Firmware Scratch Size       : 4096 B (total), 1464 B (used), 2632 B (free)
+Runtime SBI Version         : 3.0
+Standard SBI Extensions     : ipi,pmu,srst,sse,hsm,rfnc,fwft,time,base,legacy,dbcn,dbtr
+Experimental SBI Extensions : none
+
+Domain0 Name                : root
+Domain0 Boot HART           : 0
+Domain0 HARTs               : 0*
+Domain0 Region00            : 0x0000000080040000-0x000000008005ffff M: (F,R,W) S/U: ()
+Domain0 Region01            : 0x0000000080000000-0x000000008003ffff M: (F,R,X) S/U: ()
+Domain0 Region02            : 0x0000000000100000-0x0000000000100fff M: (I,R,W) S/U: (R,W)
+Domain0 Region03            : 0x0000000010000000-0x0000000010000fff M: (I,R,W) S/U: (R,W)
+Domain0 Region04            : 0x0000000002000000-0x000000000200ffff M: (I,R,W) S/U: ()
+Domain0 Region05            : 0x000000000c400000-0x000000000c5fffff M: (I,R,W) S/U: (R,W)
+Domain0 Region06            : 0x000000000c000000-0x000000000c3fffff M: (I,R,W) S/U: (R,W)
+Domain0 Region07            : 0x0000000000000000-0xffffffffffffffff M: () S/U: (R,W,X)
+Domain0 Next Address        : 0x0000000020000000
+Domain0 Next Arg1           : 0x00000000bfe00000
+Domain0 Next Mode           : S-mode
+Domain0 SysReset            : yes
+Domain0 SysSuspend          : yes
+
+Boot HART ID                : 0
+Boot HART Domain            : root
+Boot HART Priv Version      : v1.12
+Boot HART Base ISA          : rv64imafdch
+Boot HART ISA Extensions    : sstc,zicntr,zihpm,zicboz,zicbom,sdtrig,svadu
+Boot HART PMP Count         : 16
+Boot HART PMP Granularity   : 2 bits
+Boot HART PMP Address Bits  : 54
+Boot HART MHPM Info         : 16 (0x0007fff8)
+Boot HART Debug Triggers    : 2 triggers
+Boot HART MIDELEG           : 0x0000000000001666
+Boot HART MEDELEG           : 0x0000000000f4b509
+[=3hRISC-V EDK2 firmware version 2025.11-3ubuntu7
+Press ESCAPE within 5 seconds for boot options ERROR: C40000002:V03051002 I0 6D33944A-EC75-4855-A54D-809C75241F6C 83FFF850
+BdsDxe: failed to load Boot0001 "UEFI Misc Device" fr
+om PciRoot(0x0)/Pci(0x1,0x0): Not Found
+[=3hStarKernel UEFI Loader
+Loading kernel from ESP...
+[CKPT 001] Entered efi_main - ConOut live
+Monolithic build - kernel linked directly
+Collecting boot information...
+CmdLine: parsed OK
+[CKPT 004] Command line parsed
+[CKPT 005] Kernel stack allocation decided
+[CKPT 006] Boot info collected (ACPI table located)
+GOP: linear framebuffer found
+[CKPT 007] GOP: linear framebuffer found
+[CKPT 008] About to enter ExitBootServices retry loop
+Calling kernel_main (monolithic)...
+riscv64 4.3.5a probe: satp state at kernel entry
+  satp.MODE = 0x000000000000000a
+  satp.PPN  = 0x00000000000bf868
+  __kernel_start = 0x00000000bdd56982
+
+
+   _____ _             _  __                    _ 
+  / ____| |           | |/ /                   | |
+ | (___ | |_ __ _ _ __| ' / ___ _ __ _ __   ___| |
+  \___ \| __/ _` | '__|  < / _ \ '__| '_ \ / _ \ |
+  ____) | || (_| | |  | . \  __/ |  | | | |  __/ |
+ |_____/ \__\__,_|_|  |_|\_\___|_|  |_| |_|\___|_|
+
+LithosAnanke v1.5.4
+Architecture: riscv64
+Build: Aug  8 2026 10:19:17
+
+UEFI BootServices: EXITED
+
+=== StarKernel Boot Information ===
+Memory map entries: 78
+Total memory: 1020 MB
+Usable memory: 975 MB
+===================================
+
+PMM initialized.
+PMM statistics:
+  Total pages: 250357
+  Free pages : 249705
+  Used pages : 652
+  Total MB   : 977
+  Free  MB   : 975
+  Used  MB   : 2
+
+VMM initialized (mapped RAM, CR3 switched)
+VMM initialized (mapped RAM, CR3 switched)
+VMM self-test: mapped OK at 0xffff800000000000
+VMM self-test complete.
+
+IDT installed.
+
+APIC: init...
+*** EXCEPTION (riscv64) ***
+  scause = 0x000000000000000f
+  sepc   = 0x00000000bdd567f0
+  stval  = 0x000000000c201000