LoongArch: KVM: Add delay until timer interrupt injected

When timer is fired in oneshot mode, CSR.TVAL will stop with value -1
rather than 0. However when the register CSR.TVAL is restored, it will
continue to count down rather than stop there.

Now the method is to write 0 to CSR.TVAL, wait to count down for 1 cycle
at least, which is 10ns with a timer freq 100MHz, and then retore timer
interrupt status. Here add 2 cycles delay to assure that timer interrupt
is injected.

With this patch, timer selftest case passes to run always.

Cc: stable@vger.kernel.org
Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/kvm/timer.c

@@ -4,6 +4,7 @@
  */
 
 #include <linux/kvm_host.h>
+#include <asm/delay.h>
 #include <asm/kvm_csr.h>
 #include <asm/kvm_vcpu.h>
 
@@ -95,6 +96,7 @@ void kvm_restore_timer(struct kvm_vcpu *vcpu)
 		 * and set CSR TVAL with -1
 		 */
 		write_gcsr_timertick(0);
+		__delay(2); /* Wait cycles until timer interrupt injected */
 
 		/*
 		 * Writing CSR_TINTCLR_TI to LOONGARCH_CSR_TINTCLR will clear