riscv: Only send remote fences when some other CPU is online

If no other CPU is online, a local cache or TLB flush is sufficient.
These checks can be constant-folded when SMP is disabled.
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20240327045035.368512-6-samuel.holland@sifive.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/mm/cacheflush.c

@@ -21,7 +21,9 @@ void flush_icache_all(void)
 {
 	local_flush_icache_all();
 
-	if (riscv_use_sbi_for_rfence())
+	if (num_online_cpus() < 2)
+		return;
+	else if (riscv_use_sbi_for_rfence())
 		sbi_remote_fence_i(NULL);
 	else
 		on_each_cpu(ipi_remote_fence_i, NULL, 1);

arch/riscv/mm/tlbflush.c

@@ -79,7 +79,9 @@ static void __ipi_flush_tlb_all(void *info)
 
 void flush_tlb_all(void)
 {
-	if (riscv_use_sbi_for_rfence())
+	if (num_online_cpus() < 2)
+		local_flush_tlb_all();
+	else if (riscv_use_sbi_for_rfence())
 		sbi_remote_sfence_vma_asid(NULL, 0, FLUSH_TLB_MAX_SIZE, FLUSH_TLB_NO_ASID);
 	else
 		on_each_cpu(__ipi_flush_tlb_all, NULL, 1);