Re: [PATCH v7 04/15] KVM: vmx/pmu: Emulate MSR_ARCH_LBR_DEPTH for guest Arch LBR

[Like Xu <like.xu.linux@gmail.com>]

On 10/8/2021 3:38 pm, Yang Weijiang wrote:
> On Mon, Aug 09, 2021 at 09:16:47PM +0800, Like Xu wrote:
>> On 6/8/2021 3:42 pm, Yang Weijiang wrote:
>>> From: Like Xu <like.xu@linux.intel.com>
>>
>> ...
>>
>>>
>>> The number of Arch LBR entries available is determined by the value
>>> in host MSR_ARCH_LBR_DEPTH.DEPTH. The supported LBR depth values are
>>> enumerated in CPUID.(EAX=01CH, ECX=0):EAX[7:0]. For each bit "n" set
>>> in this field, the MSR_ARCH_LBR_DEPTH.DEPTH value of "8*(n+1)" is
>>> supported.
>>>
>>> On a guest write to MSR_ARCH_LBR_DEPTH, all LBR entries are reset to 0.
>>> KVM writes guest requested value to the native ARCH_LBR_DEPTH MSR
>>> (this is safe because the two values will be the same) when the Arch LBR
>>> records MSRs are pass-through to the guest.
>>>
>>> Signed-off-by: Like Xu <like.xu@linux.intel.com>
>>> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
>>> ---
>>>   arch/x86/kvm/vmx/pmu_intel.c | 35 ++++++++++++++++++++++++++++++++++-
>>>   1 file changed, 34 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
>>> index 9efc1a6b8693..a4ef5bbce186 100644
>>> --- a/arch/x86/kvm/vmx/pmu_intel.c
>>> +++ b/arch/x86/kvm/vmx/pmu_intel.c
>>> @@ -211,7 +211,7 @@ static bool intel_pmu_is_valid_lbr_msr(struct kvm_vcpu *vcpu, u32 index)
>>>   static bool intel_is_valid_msr(struct kvm_vcpu *vcpu, u32 msr)
>>>   {
>>>   	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
>>> -	int ret;
>>> +	int ret = 0;
>>>   	switch (msr) {
>>>   	case MSR_CORE_PERF_FIXED_CTR_CTRL:
>>> @@ -220,6 +220,10 @@ static bool intel_is_valid_msr(struct kvm_vcpu *vcpu, u32 msr)
>>>   	case MSR_CORE_PERF_GLOBAL_OVF_CTRL:
>>>   		ret = pmu->version > 1;
>>>   		break;
>>> +	case MSR_ARCH_LBR_DEPTH:
>>> +		if (kvm_cpu_cap_has(X86_FEATURE_ARCH_LBR))
>>> +			ret = guest_cpuid_has(vcpu, X86_FEATURE_ARCH_LBR);
>>> +		break;
>>>   	default:
>>>   		ret = get_gp_pmc(pmu, msr, MSR_IA32_PERFCTR0) ||
>>>   			get_gp_pmc(pmu, msr, MSR_P6_EVNTSEL0) ||
>>> @@ -348,10 +352,28 @@ static bool intel_pmu_handle_lbr_msrs_access(struct kvm_vcpu *vcpu,
>>>   	return true;
>>>   }
>>> +/*
>>> + * Check if the requested depth value the same as that of host.
>>> + * When guest/host depth are different, the handling would be tricky,
>>> + * so now only max depth is supported for both host and guest.
>>> + */
>>> +static bool arch_lbr_depth_is_valid(struct kvm_vcpu *vcpu, u64 depth)
>>> +{
>>> +	unsigned int eax, ebx, ecx, edx;
>>> +
>>> +	if (!kvm_cpu_cap_has(X86_FEATURE_ARCH_LBR))
>>> +		return false;
>>> +
>>> +	cpuid_count(0x1c, 0, &eax, &ebx, &ecx, &edx);
>>
>> I really don't understand why the sanity check of the
>> guest lbr depth needs to read the host's cpuid entry and it's pretty slow.
>>
> This is to address a concern from Jim:
> "Does this imply that, when restoring a vCPU, KVM_SET_CPUID2 must be called before
> KVM_SET_MSRS, so that arch_lbr_depth_is_valid() knows what to do? Is this documented
> anywhere?"

What will KVM do if the #GP behaviour of msr does not match the CPUID it is set to?

For user space host_initiated path, we may check it with the host one but
for the guest emulation, it should rely on guest cpuid, just like what we do in 
the intel_is_valid_msr().

> anyway, setting depth MSR shouldn't be hot path.

Not at all, it will be used to reset LBR entries
and it's as frequent as task switching.

>   
>> KVM has reported the maximum host LBR depth as the only supported value.
>>
>>> +
>>> +	return (depth == fls(eax & 0xff) * 8);
>>> +}
>>> +
>>>   static int intel_pmu_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>>   {
>>>   	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
>>>   	struct kvm_pmc *pmc;
>>> +	struct lbr_desc *lbr_desc = vcpu_to_lbr_desc(vcpu);
>>>   	u32 msr = msr_info->index;
>>>   	switch (msr) {
>>> @@ -367,6 +389,9 @@ static int intel_pmu_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>>   	case MSR_CORE_PERF_GLOBAL_OVF_CTRL:
>>>   		msr_info->data = pmu->global_ovf_ctrl;
>>>   		return 0;
>>> +	case MSR_ARCH_LBR_DEPTH:
>>> +		msr_info->data = lbr_desc->records.nr;
>>> +		return 0;
>>>   	default:
>>>   		if ((pmc = get_gp_pmc(pmu, msr, MSR_IA32_PERFCTR0)) ||
>>>   		    (pmc = get_gp_pmc(pmu, msr, MSR_IA32_PMC0))) {
>>> @@ -393,6 +418,7 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>>   {
>>>   	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
>>>   	struct kvm_pmc *pmc;
>>> +	struct lbr_desc *lbr_desc = vcpu_to_lbr_desc(vcpu);
>>>   	u32 msr = msr_info->index;
>>>   	u64 data = msr_info->data;
>>> @@ -427,6 +453,13 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>>   			return 0;
>>>   		}
>>>   		break;
>>> +	case MSR_ARCH_LBR_DEPTH:
>>> +		if (!arch_lbr_depth_is_valid(vcpu, data))
>>> +			return 1;
>>> +		lbr_desc->records.nr = data;
>>> +		if (!msr_info->host_initiated)
>>> +			wrmsrl(MSR_ARCH_LBR_DEPTH, lbr_desc->records.nr);
>>
>> Resetting the host msr here is dangerous,
>> what if the guest LBR event doesn't exist or isn't scheduled on?
> Hmm, should be vmcs_write to the DEPTH field, thanks for pointing this
> out!

Seriously?

>>
>>> +		return 0;
>>>   	default:
>>>   		if ((pmc = get_gp_pmc(pmu, msr, MSR_IA32_PERFCTR0)) ||
>>>   		    (pmc = get_gp_pmc(pmu, msr, MSR_IA32_PMC0))) {
>>>