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/* SPDX-License-Identifier: GPL-2.0-or-later */
/* Copyright (C) 2026 Richard Qin */
/*
* RISC-V Architecture Configuration — runtime state and KASLR
*
* Provides the runtime (post-KASLR) values of the higher-half
* virtual address bases. kaslr_init() is called once during
* early boot, before vmm_init(), to slide these bases by a
* random, 1GB-aligned offset.
*
* KASLR slide budget:
* The default Direct Map starts at 0xFFFFFFC0_0000_0000 (VPN[2]=256).
* The default Vmemmap starts at 0xFFFFFFD0_0000_0000 (VPN[2]=320).
* A 1GB superpage occupies one VPN[2] slot. We allow a slide of
* 0..63 slots (0..63 GB) downward, which keeps both regions inside
* the upper canonical half and avoids colliding with the kernel
* text region (linked at VPN[2] = 258 + 2 = ~0xFFFFFFC0_8020_0000).
*
* In practice, with 128MB RAM the kernel+direct map only touches
* VPN[2] 258-259, so a slide of up to 63GB is safe.
*/
#include <arch/config.h>
#include <lume/random.h>
#include <lume/kprintf.h>
namespace arch {
/* Runtime bases — default to the compile-time constants. */
uint64 g_direct_map_base = kDirectMapBaseDefault;
uint64 g_vmemmap_base = kVmemmapBaseDefault;
void kaslr_init()
{
#ifdef CONFIG_KASLR
/*
* Generate a random 1GB-aligned slide.
*
* Strategy (matching Linux's approach):
* 1. Pull 64 random bits from the early PRNG.
* 2. Reduce to a slot number in [0, kMaxSlots).
* 3. Convert to a byte offset (slot * 1GB).
* 4. Add to both bases (slide upward in VA space).
*
* Why 1GB alignment?
* SV39 L2 (root) PTEs map 1GB superpages. Keeping the slide
* 1GB-aligned means the direct-map can still use gigapages,
* avoiding TLB pressure. This matches Linux's KASLR on RISC-V.
*/
constexpr uint64 k1GB = 1ULL << 30;
constexpr uint64 kMaxSlots = 64; /* 0..63 GB slide range */
uint64 raw = get_random_u64();
uint64 slot = raw % kMaxSlots;
uint64 slide = slot * k1GB;
g_direct_map_base = kDirectMapBaseDefault + slide;
g_vmemmap_base = kVmemmapBaseDefault + slide;
kprintf("[kaslr] slide = %llu GB (slot %llu/64)\n", slide >> 30, slot);
kprintf("[kaslr] direct_map_base = 0x%llx\n", g_direct_map_base);
kprintf("[kaslr] vmemmap_base = 0x%llx\n", g_vmemmap_base);
#else
kprintf("[kaslr] CONFIG_KASLR disabled, using fixed layout\n");
#endif
}
} // namespace arch
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