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Message-Id: <20230810072906.4007-1-shijie@os.amperecomputing.com>
Date: Thu, 10 Aug 2023 15:29:06 +0800
From: Huang Shijie <shijie@...amperecomputing.com>
To: maz@...nel.org
Cc: oliver.upton@...ux.dev, james.morse@....com,
suzuki.poulose@....com, yuzenghui@...wei.com,
catalin.marinas@....com, will@...nel.org,
linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
linux-kernel@...r.kernel.org, patches@...erecomputing.com,
zwang@...erecomputing.com,
Huang Shijie <shijie@...amperecomputing.com>
Subject: [PATCH v2] KVM/arm64: reconfigurate the event filters for guest context
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@...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
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