compel/arch/riscv64: properly implement compel_task_size()

We need to dynamically calculate TASK_SIZE depending
on the MMU on RISC-V system. [We are using analogical
approach on aarch64/ppc64le.]

This change was tested on physical machine:
StarFive VisionFive 2
isa		: rv64imafdc_zicntr_zicsr_zifencei_zihpm_zca_zcd_zba_zbb
mmu		: sv39
uarch		: sifive,u74-mc
mvendorid	: 0x489
marchid		: 0x8000000000000007
mimpid		: 0x4210427
hart isa	: rv64imafdc_zicntr_zicsr_zifencei_zihpm_zca_zcd_zba_zbb

Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>

compel/arch/riscv64/src/lib/infect.c

@@ -181,20 +181,22 @@ int arch_fetch_sas(struct parasite_ctl *ctl, struct rt_sigframe *s)
  * Task size is the maximum virtual address space size that a process can occupy in the memory
  * Refer to linux kernel arch/riscv/include/asm/pgtable.h,
  * task size is:
- * -     0x9fc00000 (~2.5GB) for RV32.
- * -   0x4000000000 ( 256GB) for RV64 using SV39 mmu
- * - 0x800000000000 ( 128TB) for RV64 using SV48 mmu
- *
- * Note that PGDIR_SIZE must evenly divide TASK_SIZE since "RISC-V
- * Instruction Set Manual Volume II: Privileged Architecture" states that
- * "load and store effective addresses, which are 64bits, must have bits
- * 63–48 all equal to bit 47, or else a page-fault exception will occur."
-*/
-#define TASK_SIZE 0x800000000000UL // hardcoded for SV48 MMU
+ * -        0x9fc00000	(~2.5GB) for RV32.
+ * -      0x4000000000	( 256GB) for RV64 using SV39 mmu
+ * -    0x800000000000	( 128TB) for RV64 using SV48 mmu
+ * - 0x100000000000000	(  64PB) for RV64 using SV57 mmu
+ */
+#define TASK_SIZE_MIN (1UL << 38)
+#define TASK_SIZE_MAX (1UL << 56)
 
 unsigned long compel_task_size(void)
 {
-	return TASK_SIZE;
+	unsigned long task_size;
+
+	for (task_size = TASK_SIZE_MIN; task_size < TASK_SIZE_MAX; task_size <<= 1)
+		if (munmap((void *)task_size, page_size()))
+			break;
+	return task_size;
 }
 
 /*