Re: [PATCH v2 14/15] KVM: x86: Add Arch LBR data MSR access interface

Yang, Weijiang
2023-01-30T12:46:54Z

On 1/28/2023 6:13 AM, Sean Christopherson wrote:
> On Thu, Nov 24, 2022, Yang Weijiang wrote:
>> Arch LBR MSRs are xsave-supported, but they're operated as "independent"
>> xsave feature by PMU code, i.e., during thread/process context switch,
>> the MSRs are saved/restored with perf_event_task_sched_{in|out} instead
>> of generic kernel fpu switch code, i.e.,save_fpregs_to_fpstate() and
>> restore_fpregs_from_fpstate(). When vcpu guest/host fpu state swap happens,
>> Arch LBR MSRs are retained so they can be accessed directly.
>>
>> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
>> Reviewed-by: Kan Liang <kan.liang@linux.intel.com>
>> ---
>> arch/x86/kvm/vmx/pmu_intel.c | 10 ++++++++++
>> 1 file changed, 10 insertions(+)
>>
>> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
>> index b57944d5e7d8..241128972776 100644
>> --- a/arch/x86/kvm/vmx/pmu_intel.c
>> +++ b/arch/x86/kvm/vmx/pmu_intel.c
>> @@ -410,6 +410,11 @@ static int intel_pmu_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>> msr_info->data = vmcs_read64(GUEST_IA32_LBR_CTL);
>> }
>> return 0;
>> + case MSR_ARCH_LBR_FROM_0 ... MSR_ARCH_LBR_FROM_0 + 31:
>> + case MSR_ARCH_LBR_TO_0 ... MSR_ARCH_LBR_TO_0 + 31:
>> + case MSR_ARCH_LBR_INFO_0 ... MSR_ARCH_LBR_INFO_0 + 31:
>> + rdmsrl(msr_info->index, msr_info->data);
> I don't see how this is correct. As called out in patch 5:
>
> : If for some magical reason it's safe to access arch LBR MSRs without disabling
> : IRQs and confirming perf event ownership, I want to see a very detailed changelog
> : explaining exactly how that magic works.

The MSR lists here are just for live migration. When arch-lbr is active,
these MSRs are passed through

to guest.

>
>> + return 0;
>> default:
>> if ((pmc = get_gp_pmc(pmu, msr, MSR_IA32_PERFCTR0)) ||
>> (pmc = get_gp_pmc(pmu, msr, MSR_IA32_PMC0))) {
>> @@ -528,6 +533,11 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>> (data & ARCH_LBR_CTL_LBREN))
>> intel_pmu_create_guest_lbr_event(vcpu);
>> return 0;
>> + case MSR_ARCH_LBR_FROM_0 ... MSR_ARCH_LBR_FROM_0 + 31:
>> + case MSR_ARCH_LBR_TO_0 ... MSR_ARCH_LBR_TO_0 + 31:
>> + case MSR_ARCH_LBR_INFO_0 ... MSR_ARCH_LBR_INFO_0 + 31:
>> + wrmsrl(msr_info->index, msr_info->data);
>> + return 0;
>> default:
>> if ((pmc = get_gp_pmc(pmu, msr, MSR_IA32_PERFCTR0)) ||
>> (pmc = get_gp_pmc(pmu, msr, MSR_IA32_PMC0))) {
>> --
>> 2.27.0
>>