linux-kernel - Re: [PATCH RFC] Revert "KVM: VMX: Micro-optimize vmexit time when not exposing PMU"

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Date:   Fri, 19 Jun 2020 14:13:52 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>
Cc:     Jim Mattson <jmattson@...gle.com>,
        Maxime Coquelin <maxime.coquelin@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] Revert "KVM: VMX: Micro-optimize vmexit time when not
 exposing PMU"

On 19/06/20 11:40, Vitaly Kuznetsov wrote:
> Guest crashes are observed on a Cascade Lake system when 'perf top' is
> launched on the host, e.g.
> 
>  BUG: unable to handle kernel paging request at fffffe0000073038
>  PGD 7ffa7067 P4D 7ffa7067 PUD 7ffa6067 PMD 7ffa5067 PTE ffffffffff120
>  Oops: 0000 [#1] SMP PTI
>  CPU: 1 PID: 1 Comm: systemd Not tainted 4.18.0+ #380
> ...
>  Call Trace:
>   serial8250_console_write+0xfe/0x1f0
>   call_console_drivers.constprop.0+0x9d/0x120
>   console_unlock+0x1ea/0x460
> 
> Call traces are different but the crash is imminent. The problem was
> blindly bisected to the commit 041bc42ce2d0 ("KVM: VMX: Micro-optimize
> vmexit time when not exposing PMU"). It was also confirmed that the
> issue goes away if PMU is exposed to the guest.
> 
> With some instrumentation of the guest we can see what is being switched
> (when we do atomic_switch_perf_msrs()):
> 
>  vmx_vcpu_run: switching 2 msrs
>  vmx_vcpu_run: switching MSR38f guest: 70000000d host: 70000000f
>  vmx_vcpu_run: switching MSR3f1 guest: 0 host: 2
> 
> The current guess is that PEBS (MSR_IA32_PEBS_ENABLE, 0x3f1) is to blame.
> Regardless of whether PMU is exposed to the guest or not, PEBS needs to
> be disabled upon switch.
> 
> This reverts commit 041bc42ce2d0efac3b85bbb81dea8c74b81f4ef9.
> 
> Reported-by: Maxime Coquelin <maxime.coquelin@...hat.com>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
> - Perf/KVM interractions are a mystery to me, thus RFC.
> ---
>  arch/x86/kvm/vmx/vmx.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 36c771728c8c..b1a23ad986ff 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6728,8 +6728,7 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
>  
>  	pt_guest_enter(vmx);
>  
> -	if (vcpu_to_pmu(vcpu)->version)
> -		atomic_switch_perf_msrs(vmx);
> +	atomic_switch_perf_msrs(vmx);
>  	atomic_switch_umwait_control_msr(vmx);
>  
>  	if (enable_preemption_timer)
> 

Queued, thanks.

Paolo