On 2021/1/26 17:42, Paolo Bonzini wrote:
On 08/01/21 02:36, Like Xu wrote:
@@ -401,6 +398,9 @@ static void intel_pmu_init(struct kvm_vcpu *vcpu)
pmu->fixed_counters[i].idx = i + INTEL_PMC_IDX_FIXED;
pmu->fixed_counters[i].current_config = 0;
}
+
+ vcpu->arch.perf_capabilities = guest_cpuid_has(vcpu,
X86_FEATURE_PDCM) ?
+ vmx_get_perf_capabilities() : 0;
There is one thing I don't understand with this patch: intel_pmu_init is
not called when CPUID is changed. So I would have thought that anything
that uses guest_cpuid_has must stay in intel_pmu_refresh. As I understand
it vcpu->arch.perf_capabilities is always set to 0
(vmx_get_perf_capabilities is never called), and kvm_set_msr_common would
fail to set any bit in the MSR. What am I missing?
In addition, the code of patch 4:
+ if (!intel_pmu_lbr_is_enabled(vcpu)) {
+ vcpu->arch.perf_capabilities &= ~PMU_CAP_LBR_FMT;
+ lbr_desc->records.nr = 0;
+ }
is not okay after MSR changes. The value written by the host must be
either rejected (with "return 1") or applied unchanged.
Fortunately I think this code is dead if you move the check in kvm_set_msr
from patch 9 to patch 4. However, in patch 9 vmx_get_perf_capabilities()
must only set the LBR format bits if intel_pmu_lbr_is_compatible(vcpu).
Thanks for the guidance. How about handling it in this way:
In the intel_pmu_init():
vcpu->arch.perf_capabilities = 0;
lbr_desc->records.nr = 0;
In the intel_pmu_refresh():
if (guest_cpuid_has(vcpu, X86_FEATURE_PDCM)) {
vcpu->arch.perf_capabilities = vmx_get_perf_capabilities();
if (!lbr_desc->records.nr)
vcpu->arch.perf_capabilities &= ~PMU_CAP_LBR_FMT;
}
In the vmx_set_msr():
case MSR_IA32_PERF_CAPABILITIES:
// set up lbr_desc->records.nr
if (!intel_pmu_lbr_is_compatible(vcpu))
return 1;
ret = kvm_set_msr_common(vcpu, msr_info);
In the kvm_set_msr_common():
case MSR_IA32_PERF_CAPABILITIES:
vcpu->arch.perf_capabilities = data;
kvm_pmu_refresh(vcpu);
The patches look good apart from these issues and the other nits I pointed
out. However, you need testcases here, for both kvm-unit-tests and
tools/testing/selftests/kvm.
For KVM, it would be at least a basic check that looks for the MSR LBR
(using the MSR indices for the various processors), does a branch, and
checks that the FROM_IP/TO_IP are good. You can write the kvm-unit-tests
using the QEMU option "-cpu host,migratable=no": if you do this, QEMU will
pick the KVM_GET_SUPPORTED_CPUID bits and move them more or less directly
into the guest CPUID.
For tools/testing/selftests/kvm, your test need to check the effect of
various CPUID settings on the PERF_CAPABILITIES MSR, check that whatever
you write with KVM_SET_MSR is _not_ modified and can be retrieved with
KVM_GET_MSR, and check that invalid LBR formats are rejected.
Thanks, I will add the above tests in the next version.
I'm really, really sorry for leaving these patches on my todo list for
months, but you guys need to understand the main reason for this: they come
with no testcases. A large patch series adding userspace APIs and
complicated CPUID/MSR processing *automatically* goes to the bottom of my
queue, because:
- I need to go with a fine comb over all the userspace API changes, I
cannot just look at test code and see if it works.
- I will have no way to test its correctness after it's committed.
For you, the work ends when your patch is accepted. For me, that's when
the work begins, and I need to make sure that the patch will be
maintainable in the future.
Thanks, and sorry again for the delay.
Paolo
