[PATCH v2] KVM/arm64: reconfigurate the event filters for guest context

[Huang Shijie]

1.) Background.
   1.1) In arm64, start a guest with Qemu which is running as a VMM of KVM,
        and bind the guest to core 33 and run program "a" in guest.
        The code of "a" shows below:
   	----------------------------------------------------------
		#include <stdio.h>

		int main()
		{
			unsigned long i = 0;

			for (;;) {
				i++;
			}

			printf("i:%ld\n", i);
			return 0;
		}
   	----------------------------------------------------------

   1.2) Use the following perf command in host:
      #perf stat -e cycles:G,cycles:H -C 33 -I 1000 sleep 1
          #           time             counts unit events
               1.000817400      3,299,471,572      cycles:G
               1.000817400          3,240,586      cycles:H

       This result is correct, my cpu's frequency is 3.3G.

   1.3) Use the following perf command in host:
      #perf stat -e cycles:G,cycles:H -C 33 -d -d  -I 1000 sleep 1
            time             counts unit events
     1.000831480        153,634,097      cycles:G                                                                (70.03%)
     1.000831480      3,147,940,599      cycles:H                                                                (70.03%)
     1.000831480      1,143,598,527      L1-dcache-loads                                                         (70.03%)
     1.000831480              9,986      L1-dcache-load-misses            #    0.00% of all L1-dcache accesses   (70.03%)
     1.000831480    <not supported>      LLC-loads
     1.000831480    <not supported>      LLC-load-misses
     1.000831480        580,887,696      L1-icache-loads                                                         (70.03%)
     1.000831480             77,855      L1-icache-load-misses            #    0.01% of all L1-icache accesses   (70.03%)
     1.000831480      6,112,224,612      dTLB-loads                                                              (70.03%)
     1.000831480             16,222      dTLB-load-misses                 #    0.00% of all dTLB cache accesses  (69.94%)
     1.000831480        590,015,996      iTLB-loads                                                              (59.95%)
     1.000831480                505      iTLB-load-misses                 #    0.00% of all iTLB cache accesses  (59.95%)

       This result is wrong. The "cycle:G" should be nearly 3.3G.

2.) Root cause.
	There is only 7 counters in my arm64 platform:
	  (one cycle counter) + (6 normal counters)

	In 1.3 above, we will use 10 event counters.
	Since we only have 7 counters, the perf core will trigger
       	multiplexing in hrtimer:
	     perf_mux_hrtimer_restart() --> perf_rotate_context().

       If the hrtimer occurs when the host is running, it's fine.
       If the hrtimer occurs when the guest is running,
       the perf_rotate_context() will program the PMU with filters for
       host context. The KVM does not have a chance to restore
       PMU registers with kvm_vcpu_pmu_restore_guest().
       The PMU does not work correctly, so we got wrong result.

3.) About this patch.
	Make a KVM_REQ_RELOAD_PMU request before reentering the
	guest. The request will call kvm_vcpu_pmu_restore_guest()
	to reconfigurate the filters for guest context.

4.) Test result of this patch:
      #perf stat -e cycles:G,cycles:H -C 33 -d -d  -I 1000 sleep 1
            time             counts unit events
     1.001006400      3,298,348,656      cycles:G                                                                (70.03%)
     1.001006400          3,144,532      cycles:H                                                                (70.03%)
     1.001006400            941,149      L1-dcache-loads                                                         (70.03%)
     1.001006400             17,937      L1-dcache-load-misses            #    1.91% of all L1-dcache accesses   (70.03%)
     1.001006400    <not supported>      LLC-loads
     1.001006400    <not supported>      LLC-load-misses
     1.001006400          1,101,889      L1-icache-loads                                                         (70.03%)
     1.001006400            121,638      L1-icache-load-misses            #   11.04% of all L1-icache accesses   (70.03%)
     1.001006400          1,031,228      dTLB-loads                                                              (70.03%)
     1.001006400             26,952      dTLB-load-misses                 #    2.61% of all dTLB cache accesses  (69.93%)
     1.001006400          1,030,678      iTLB-loads                                                              (59.94%)
     1.001006400                338      iTLB-load-misses                 #    0.03% of all iTLB cache accesses  (59.94%)

    The result is correct. The "cycle:G" is nearly 3.3G now.

Signed-off-by: Huang Shijie <shijie at os.amperecomputing.com>
---
v1 --> v2:
	Do not change perf/core code, only change the ARM64 kvm code.
	v1: https://lkml.org/lkml/2023/8/8/1465

---
 arch/arm64/kvm/arm.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index c2c14059f6a8..475a2f0e0e40 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -919,8 +919,17 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
 		if (!ret)
 			ret = 1;
 
-		if (ret > 0)
+		if (ret > 0) {
+			/*
+			 * The perf_rotate_context() may rotate the events and
+			 * reprogram PMU with filters for host context.
+			 * So make a request before reentering the guest to
+			 * reconfigurate the event filters for guest context.
+			 */
+			kvm_make_request(KVM_REQ_RELOAD_PMU, vcpu);
+
 			ret = check_vcpu_requests(vcpu);
+		}
 
 		/*
 		 * Preparing the interrupts to be injected also
-- 
2.39.2