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

[Sean Christopherson <seanjc@google.com>]

On Tue, Jan 31, 2023, Yang, Weijiang wrote:
> 
> On 1/31/2023 1:30 AM, Sean Christopherson wrote:
> > On Mon, Jan 30, 2023, Yang, Weijiang wrote:
> > > 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.
> > None of that explains how the guest's MSR values are guaranteed to be resident
> > in hardware.
> 
> I ignored host *event* scheduling case in commit log.
> 
> My understanding is, host LBR *event* could break in at any point when the
> vCPU is running,
> 
> in this case disabling IRQs before read/write the MSRs is pointless because
> the HW context could have been swapped. I need to do more investigation for
> the issue.

Which is presumably why intel_pmu_handle_lbr_msrs_access() checks that the LBR
perf event is active prior to accessing the MSRs, with IRQs disabled...

	/*
	 * Disable irq to ensure the LBR feature doesn't get reclaimed by the
	 * host at the time the value is read from the msr, and this avoids the
	 * host LBR value to be leaked to the guest. If LBR has been reclaimed,
	 * return 0 on guest reads.
	 */
	local_irq_disable();
	if (lbr_desc->event->state == PERF_EVENT_STATE_ACTIVE) {
		if (read)
			rdmsrl(index, msr_info->data);
		else
			wrmsrl(index, msr_info->data);
		__set_bit(INTEL_PMC_IDX_FIXED_VLBR, vcpu_to_pmu(vcpu)->pmc_in_use);
		local_irq_enable();
		return true;
	}
	clear_bit(INTEL_PMC_IDX_FIXED_VLBR, vcpu_to_pmu(vcpu)->pmc_in_use);
	local_irq_enable();