RISCV64: Use va_kernel_pa_offset in VTOP()

Since RISC-V Linux v6.4, the commit 3335068f8721 ("riscv: Use
PUD/P4D/PGD pages for the linear mapping") changes phys_ram_base from
the physical start of the kernel to the actual start of the DRAM.

The Crash's VTOP() still uses phys_ram_base and kernel_map.virt_addr
to translate kernel virtual address, that made Crash boot failed with
Linux v6.4 and later version.

Let Linux export kernel_map.va_kernel_pa_offset in v6.5 and backported
v6.4.0 stable, so Crash can use "va_kernel_pa_offset" to translate the
kernel virtual address in VTOP() correctly.

Signed-off-by: Song Shuai <suagrfillet@gmail.com>

defs.h

@@ -3663,8 +3663,7 @@ typedef signed int s32;
 	ulong _X = X;									\
 	(THIS_KERNEL_VERSION >= LINUX(5,13,0) &&					\
 		(_X) >= machdep->machspec->kernel_link_addr) ?				\
-		(((unsigned long)(_X)-(machdep->machspec->kernel_link_addr)) +		\
-		 machdep->machspec->phys_base):						\
+		((unsigned long)(_X)-(machdep->machspec->va_kernel_pa_offset)): 	\
 		(((unsigned long)(_X)-(machdep->kvbase)) +				\
 		 machdep->machspec->phys_base);						\
 	})
@@ -7022,6 +7021,7 @@ struct machine_specific {
 	ulong modules_vaddr;
 	ulong modules_end;
 	ulong kernel_link_addr;
+	ulong va_kernel_pa_offset;
 
 	ulong _page_present;
 	ulong _page_read;

riscv64.c

@@ -418,6 +418,28 @@ static void riscv64_get_va_range(struct machine_specific *ms)
 	error(FATAL, "cannot get vm layout\n");
 }
 
+static void
+riscv64_get_va_kernel_pa_offset(struct machine_specific *ms)
+{
+	unsigned long kernel_version = riscv64_get_kernel_version();
+
+	/*
+	 * Since Linux v6.4 phys_base is not the physical start of the kernel,
+	 * trying to use "va_kernel_pa_offset" to determine the offset between
+	 * kernel virtual and physical addresses.
+	 */
+	if (kernel_version >= LINUX(6,4,0)) {
+		char *string;
+		if ((string = pc->read_vmcoreinfo("NUMBER(va_kernel_pa_offset)"))) {
+			ms->va_kernel_pa_offset = htol(string, QUIET, NULL);
+			free(string);
+		} else
+			error(FATAL, "cannot read va_kernel_pa_offset\n");
+	}
+	else
+		ms->va_kernel_pa_offset = ms->kernel_link_addr - ms->phys_base;
+}
+
 static int
 riscv64_is_kvaddr(ulong vaddr)
 {
@@ -1352,6 +1374,7 @@ riscv64_init(int when)
 		riscv64_get_struct_page_size(machdep->machspec);
 		riscv64_get_va_bits(machdep->machspec);
 		riscv64_get_va_range(machdep->machspec);
+		riscv64_get_va_kernel_pa_offset(machdep->machspec);
 
 		pt_level_alloc(&machdep->pgd, "cannot malloc pgd space.");
 		pt_level_alloc(&machdep->machspec->p4d, "cannot malloc p4d space.");