linux-kernel - Re: [PATCH v5 05/13] KVM: vmx/pmu: Emulate MSR_ARCH_LBR

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Message-ID: <bd386ed1-69d8-39ab-8bee-6e3aed8d2ee2@gmail.com>
Date:   Mon, 12 Jul 2021 18:10:30 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Yang Weijiang <weijiang.yang@...el.com>,
        Jim Mattson <jmattson@...gle.com>
Cc:     pbonzini@...hat.com, seanjc@...gle.com, vkuznets@...hat.com,
        wei.w.wang@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Like Xu <like.xu@...ux.intel.com>
Subject: Re: [PATCH v5 05/13] KVM: vmx/pmu: Emulate MSR_ARCH_LBR_CTL for guest
 Arch LBR

On 12/7/2021 5:36 pm, Yang Weijiang wrote:
> On Fri, Jul 09, 2021 at 02:55:35PM -0700, Jim Mattson wrote:
>> On Fri, Jul 9, 2021 at 2:51 AM Yang Weijiang <weijiang.yang@...el.com> wrote:
>>>
>>> From: Like Xu <like.xu@...ux.intel.com>
>>>
>>> Arch LBRs are enabled by setting MSR_ARCH_LBR_CTL.LBREn to 1. A new guest
>>> state field named "Guest IA32_LBR_CTL" is added to enhance guest LBR usage.
>>> When guest Arch LBR is enabled, a guest LBR event will be created like the
>>> model-specific LBR does. Clear guest LBR enable bit on host PMI handling so
>>> guest can see expected config.
>>>
>>> On processors that support Arch LBR, MSR_IA32_DEBUGCTLMSR[bit 0] has no
>>> meaning. It can be written to 0 or 1, but reads will always return 0.
>>> Like IA32_DEBUGCTL, IA32_ARCH_LBR_CTL msr is also reserved on INIT.
>>
>> I suspect you mean "preserved" rather than "reserved."
> Yes, should be preserved.
> 
>>
>>> Signed-off-by: Like Xu <like.xu@...ux.intel.com>
>>> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
>>> ---
>>>   arch/x86/events/intel/lbr.c      |  2 --
>>>   arch/x86/include/asm/msr-index.h |  1 +
>>>   arch/x86/include/asm/vmx.h       |  2 ++
>>>   arch/x86/kvm/vmx/pmu_intel.c     | 31 ++++++++++++++++++++++++++-----
>>>   arch/x86/kvm/vmx/vmx.c           |  9 +++++++++
>>>   5 files changed, 38 insertions(+), 7 deletions(-)
>>>
>>
>>> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
>>> index da68f0e74702..4500c564c63a 100644
>>> --- a/arch/x86/kvm/vmx/pmu_intel.c
>>> +++ b/arch/x86/kvm/vmx/pmu_intel.c
>>> @@ -19,6 +19,11 @@
>>>   #include "pmu.h"
>>>
>>>   #define MSR_PMC_FULL_WIDTH_BIT      (MSR_IA32_PMC0 - MSR_IA32_PERFCTR0)
>>> +/*
>>> + * Regardless of the Arch LBR or legacy LBR, when the LBR_EN bit 0 of the
>>> + * corresponding control MSR is set to 1, LBR recording will be enabled.
>>> + */
>>
>> Is this comment misplaced? It doesn't seem to have anything to do with
>> the macro being defined below.
> Agree, will put this in commit message.
>>
>>> @@ -458,6 +467,14 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>>                  lbr_desc->records.nr = data;
>>>                  lbr_desc->arch_lbr_reset = true;
>>>                  return 0;
>>> +       case MSR_ARCH_LBR_CTL:
>>> +               if (data & ~KVM_ARCH_LBR_CTL_MASK)
>>
>> Is a static mask sufficient? Per the Intel® Architecture Instruction
>> Set Extensions and Future Features Programming Reference, some of
>> these bits may not be supported on all microarchitectures. See Table
>> 7-8. CPUID Leaf 01CH Enumeration of Architectural LBR Capabilities.
> Yes, more sanity checks are required, thanks!
> 
>>
>>> +                       break;
>>> +               vmcs_write64(GUEST_IA32_LBR_CTL, data);
>>> +               if (intel_pmu_lbr_is_enabled(vcpu) && !lbr_desc->event &&
>>> +                   (data & ARCH_LBR_CTL_LBREN))
>>> +                       intel_pmu_create_guest_lbr_event(vcpu);
>>
>> Nothing has to be done when the LBREN bit goes from 1 to 0?
> Need to release the event and reset related flag when the bit goes from
> 1 to 0. Thanks!

No need to release the LBR event and it will be lazily released.

>>
>>> +               return 0;
>>>          default:
>>>                  if ((pmc = get_gp_pmc(pmu, msr, MSR_IA32_PERFCTR0)) ||
>>>                      (pmc = get_gp_pmc(pmu, msr, MSR_IA32_PMC0))) {
>>
>> Per the Intel® Architecture Instruction Set Extensions and Future
>> Features Programming Reference, "IA32_LBR_CTL.LBREn is saved and
>> cleared on #SMI, and restored on RSM." I don't see that happening
>> anywhere. That manual also says, "On a warm reset...IA32_LBR_CTL.LBREn
>> is cleared to 0, disabling LBRs." I don't see that happening either.
> 
> Yes, I'll add related code to make it consistent with spec, thanks!
>>
>> I have a question about section 7.1.4.4 in that manual. It says, "On a
>> debug breakpoint event (#DB), IA32_LBR_CTL.LBREn is cleared." When,
>> exactly, does that happen? In particular, if kvm synthesizes such an
>> event (for example, in kvm_vcpu_do_singlestep), does
>> IA32_LBR_CTL.LBREn automatically get cleared (after loading the guest
>> IA32_LBR_CTL value from the VMCS)? Or does kvm need to explicitly
>> clear that bit in the VMCS before injecting the #DB?
> OK, I don't have answer now, will ask the Arch to get clear answer on this,
> thanks for raising the question!

I think we also need a kvm-unit-tests to cover it (as well as the legacy 
LBR).

>