Re: [PATCH v5 05/13] KVM: vmx/pmu: Emulate MSR_ARCH_LBR_CTL for guest Arch LBR

[Yang Weijiang]

On Mon, Jul 12, 2021 at 06:10:30PM +0800, Like Xu wrote:
> 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@xxxxxxxxx> wrote:
> >>>
> >>>From: Like Xu <like.xu@xxxxxxxxxxxxxxx>
> >>>
> >>>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@xxxxxxxxxxxxxxx>
> >>>Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> >>>---
> >>>  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.
>
I forgot the lazy cleanup :-), thanks!
 
> >>
> >>>+               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).
> 
Yes, I'll add one later.

> >