RISC-V: Issue a local tlbflush if possible.

In RISC-V, tlb flush happens via SBI which is expensive. If the local
cpu is the only cpu in cpumask, there is no need to invoke a SBI call.

Just do a local flush and return.
Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>

arch/riscv/mm/tlbflush.c

@@ -2,6 +2,7 @@
 
 #include <linux/mm.h>
 #include <linux/smp.h>
+#include <linux/sched.h>
 #include <asm/sbi.h>
 
 void flush_tlb_all(void)
@@ -9,16 +10,30 @@ void flush_tlb_all(void)
 	sbi_remote_sfence_vma(NULL, 0, -1);
 }
 
+/*
+ * This function must not be called with cmask being null.
+ * Kernel may panic if cmask is NULL.
+ */
 static void __sbi_tlb_flush_range(struct cpumask *cmask, unsigned long start,
 				  unsigned long size)
 {
 	struct cpumask hmask;
+	unsigned int cpuid;
 
 	if (cpumask_empty(cmask))
 		return;
 
-	riscv_cpuid_to_hartid_mask(cmask, &hmask);
-	sbi_remote_sfence_vma(hmask.bits, start, size);
+	cpuid = get_cpu();
+
+	if (cpumask_any_but(cmask, cpuid) >= nr_cpu_ids) {
+		/* local cpu is the only cpu present in cpumask */
+		local_flush_tlb_all();
+	} else {
+		riscv_cpuid_to_hartid_mask(cmask, &hmask);
+		sbi_remote_sfence_vma(cpumask_bits(&hmask), start, size);
+	}
+
+	put_cpu();
 }
 
 void flush_tlb_mm(struct mm_struct *mm)