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Subject: Re: [PATCH v1 1/3] KVM: VMX: Consider PID.PIR to determine if vCPU
 has pending interrupts
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Liran Alon <liran.alon@oracle.com>,
        Jag Raman <jag.raman@oracle.com>
References: <20191106175602.4515-1-joao.m.martins@oracle.com>
 <20191106175602.4515-2-joao.m.martins@oracle.com>
 <67bca655-fea3-4b57-be3c-7dc58026b5d9@redhat.com>
From:   Joao Martins <joao.m.martins@oracle.com>
Message-ID: <030dd147-8c4f-d6e3-85a8-ee743ce4d5b0@oracle.com>
Date:   Mon, 11 Nov 2019 14:59:03 +0000
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On 11/11/19 2:46 PM, Paolo Bonzini wrote:
> On 06/11/19 18:56, Joao Martins wrote:
>> Commit 17e433b54393 ("KVM: Fix leak vCPU's VMCS value into other pCPU")
>> introduced vmx_dy_apicv_has_pending_interrupt() in order to determine
>> if a vCPU have a pending posted interrupt. This routine is used by
>> kvm_vcpu_on_spin() when searching for a a new runnable vCPU to schedule
>> on pCPU instead of a vCPU doing busy loop.
>>
>> vmx_dy_apicv_has_pending_interrupt() determines if a
>> vCPU has a pending posted interrupt solely based on PID.ON. However,
>> when a vCPU is preempted, vmx_vcpu_pi_put() sets PID.SN which cause
>> raised posted interrupts to only set bit in PID.PIR without setting
>> PID.ON (and without sending notification vector), as depicted in VT-d
>> manual section 5.2.3 "Interrupt-Posting Hardware Operation".
>>
>> Therefore, checking PID.ON is insufficient to determine if a vCPU has
>> pending posted interrupts and instead we should also check if there is
>> some bit set on PID.PIR.
>>
>> Fixes: 17e433b54393 ("KVM: Fix leak vCPU's VMCS value into other pCPU")
>> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
>> Signed-off-by: Liran Alon <liran.alon@oracle.com>
>> ---
>>  arch/x86/kvm/vmx/vmx.c | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index 31ce6bc2c371..18b0bee662a5 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -6141,7 +6141,10 @@ static int vmx_sync_pir_to_irr(struct kvm_vcpu *vcpu)
>>  
>>  static bool vmx_dy_apicv_has_pending_interrupt(struct kvm_vcpu *vcpu)
>>  {
>> -	return pi_test_on(vcpu_to_pi_desc(vcpu));
>> +	struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);
>> +
>> +	return pi_test_on(pi_desc) ||
>> +		!bitmap_empty((unsigned long *)pi_desc->pir, NR_VECTORS);
>>  }
>>  
>>  static void vmx_load_eoi_exitmap(struct kvm_vcpu *vcpu, u64 *eoi_exit_bitmap)
> 
> Should we check the bitmap only if SN is false?  We have a precondition
> that if SN is clear then non-empty PIR implies ON=1 (modulo the small
> window in vmx_vcpu_pi_load of course), so that'd be a bit faster.

Makes sense;

The bitmap check was really meant for SN=1.

Should SN=0 we would be saving ~22-27 cycles as far as I micro-benchmarked a few
weeks ago. Now that you suggest it, it would be also good for older platforms too.

Cheers,
	Joao