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Message-ID: <gsntmsfl2rch.fsf@coltonlewis-kvm.c.googlers.com>
Date: Mon, 20 Jan 2025 19:54:54 +0000
From: Colton Lewis <coltonlewis@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: kvm@...r.kernel.org, mizhang@...gle.com, ljr.kernel@...il.com,
jmattson@...gle.com, aaronlewis@...gle.com, pbonzini@...hat.com,
shuah@...nel.org, linux-kselftest@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/6] KVM: x86: selftests: Test read/write core counters
Sean Christopherson <seanjc@...gle.com> writes:
> On Wed, Sep 18, 2024, Colton Lewis wrote:
>> Run a basic test to ensure we can write an arbitrary value to the core
>> counters and read it back.
>> Signed-off-by: Colton Lewis <coltonlewis@...gle.com>
>> ---
>> .../selftests/kvm/x86_64/pmu_counters_test.c | 54 +++++++++++++++++++
>> 1 file changed, 54 insertions(+)
>> diff --git a/tools/testing/selftests/kvm/x86_64/pmu_counters_test.c
>> b/tools/testing/selftests/kvm/x86_64/pmu_counters_test.c
>> index 5b240585edc5..79ca7d608e00 100644
>> --- a/tools/testing/selftests/kvm/x86_64/pmu_counters_test.c
>> +++ b/tools/testing/selftests/kvm/x86_64/pmu_counters_test.c
>> @@ -641,11 +641,65 @@ static uint8_t nr_core_counters(void)
>> return AMD_NR_CORE_EXT_COUNTERS;
>> return AMD_NR_CORE_COUNTERS;
>> +}
>> +
>> +static uint8_t guest_nr_core_counters(void)
>> +{
>> + uint8_t nr_counters =
>> this_cpu_property(X86_PROPERTY_NUM_PERF_CTR_CORE);
>> + bool core_ext = this_cpu_has(X86_FEATURE_PERF_CTR_EXT_CORE);
> For both this and nr_core_counters(), there's no need to read
> PERF_CTR_EXT_CORE
> if nr_counters is non-zero, and then no need to capture it in a local
> variable.
Sure but since I might need it and don't see why the performance cost
matters for a test that is only calling it a few times, I thought the
code looked nicer to just read it up front when I declare the variable.
I can change it.
>> +
>> + if (nr_counters != 0)
>> + return nr_counters;
>> +
>> + if (core_ext)
>> + return AMD_NR_CORE_EXT_COUNTERS;
>> +
>> + return AMD_NR_CORE_COUNTERS;
> This is *painfully* similar to nr_core_counters(). It actually took me
> almost
> a minute of staring to see the difference. One option would be to add a
> helper
> to dedup the if-statements, but while somewhat gross, I actually think a
> macro
> is the way to go.
> #define nr_core_counters(scope) \
> ({ \
> uint8_t nr_counters =
> scope##_cpu_property(X86_PROPERTY_NR_PERFCTR_CORE); \
> \
> if (!nr_counters) { \
> if (scope##_cpu_has(X86_FEATURE_PERFCTR_CORE)) \
> nr_counters = AMD_NR_CORE_EXT_COUNTERS; \
> else \
> nr_counters = AMD_NR_CORE_COUNTERS; \
> } \
> nr_counters; \
> })
> static uint8_t kvm_nr_core_counters(void)
> {
> return nr_core_counters(kvm);
> }
> static uint8_t guest_nr_core_counters(void)
> {
> return nr_core_counters(this);
> }
Point taken. I'll go with the macro.
>> +
> Unnecessary newline.
Will delete
>> +}
>> +static void guest_test_rdwr_core_counters(void)
>> +{
>> + bool core_ext = this_cpu_has(X86_FEATURE_PERF_CTR_EXT_CORE);
>> + uint8_t nr_counters = guest_nr_core_counters();
>> + uint8_t i;
>> + uint32_t esel_msr_base = core_ext ? MSR_F15H_PERF_CTL :
>> MSR_K7_EVNTSEL0;
> Please don't concoct new abbreviations. "esel" isn't used anywhere in
> KVM, and
> AFAICT it's not used in perf either.
I'll avoid that in the future
> I would also prefer to have consistent naming between the Intel and AMD
> tests
> (the Intel test uses base_<name>_msr).
Done
> base_eventsel_msr is all of four characters more.
>> + uint32_t cnt_msr_base = core_ext ? MSR_F15H_PERF_CTR : MSR_K7_PERFCTR0;
> For better or worse, the Intel version uses "base_pmc_msr". I see no
> reason to
> diverage from that.
Done
>> + uint32_t msr_step = core_ext ? 2 : 1;
>> +
>> + for (i = 0; i < AMD_NR_CORE_EXT_COUNTERS; i++) {
>> + uint64_t test_val = 0xffff;
>> + uint32_t esel_msr = esel_msr_base + msr_step * i;
>> + uint32_t cnt_msr = cnt_msr_base + msr_step * i;
> And then
> uint32_t eventsel_msr = ...;
> uint32_t pmc_msr = ...;
>> + bool expect_gp = !(i < nr_counters);
> Uh, isn't that just a weird way of writing:
> bool expect_gp = i >= nr_counters;
Yes they are logically equivalent. I thought it was clearer by
emphasizing it was the negation of "i is a valid counter" (i <
nr_counters)
But I'll change it
>> + uint8_t vector;
>> + uint64_t val;
>> +
>> + /* Test event selection register. */
> This is pretty obvious if the MSR is named eventsel_msr.
Will delete
>> + vector = wrmsr_safe(esel_msr, test_val);
>> + GUEST_ASSERT_PMC_MSR_ACCESS(WRMSR, esel_msr, expect_gp, vector);
>> +
>> + vector = rdmsr_safe(esel_msr, &val);
>> + GUEST_ASSERT_PMC_MSR_ACCESS(RDMSR, esel_msr, expect_gp, vector);
>> +
>> + if (!expect_gp)
>> + GUEST_ASSERT_PMC_VALUE(RDMSR, esel_msr, val, test_val);
>> +
>> + /* Test counter register. */
> Same thing here. If there is novel information/behavior, then by all
> means add
> a comment.
Will delete
>> + vector = wrmsr_safe(cnt_msr, test_val);
>> + GUEST_ASSERT_PMC_MSR_ACCESS(WRMSR, cnt_msr, expect_gp, vector);
>> +
>> + vector = rdmsr_safe(cnt_msr, &val);
>> + GUEST_ASSERT_PMC_MSR_ACCESS(RDMSR, cnt_msr, expect_gp, vector);
>> +
>> + if (!expect_gp)
>> + GUEST_ASSERT_PMC_VALUE(RDMSR, cnt_msr, val, test_val);
>> + }
>> }
>> static void guest_test_core_counters(void)
>> {
>> + guest_test_rdwr_core_counters();
>> GUEST_DONE();
>> }
>> --
>> 2.46.0.662.g92d0881bb0-goog
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