linux-kernel - Re: [PATCH v1 2/3] KVM: VMX: Do not change PID.NDST when loading a blocked vCPU

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Date:   Mon, 11 Nov 2019 15:39:07 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Joao Martins <joao.m.martins@...cle.com>, kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Liran Alon <liran.alon@...cle.com>,
        Jag Raman <jag.raman@...cle.com>
Subject: Re: [PATCH v1 2/3] KVM: VMX: Do not change PID.NDST when loading a
 blocked vCPU

On 06/11/19 18:56, Joao Martins wrote:
> When vCPU enters block phase, pi_pre_block() inserts vCPU to a per pCPU
> linked list of all vCPUs that are blocked on this pCPU. Afterwards, it
> changes PID.NV to POSTED_INTR_WAKEUP_VECTOR which its handler
> (wakeup_handler()) is responsible to kick (unblock) any vCPU on that
> linked list that now has pending posted interrupts.
> 
> While vCPU is blocked (in kvm_vcpu_block()), it may be preempted which
> will cause vmx_vcpu_pi_put() to set PID.SN.  If later the vCPU will be
> scheduled to run on a different pCPU, vmx_vcpu_pi_load() will clear
> PID.SN but will also *overwrite PID.NDST to this different pCPU*.
> Instead of keeping it with original pCPU which vCPU had entered block
> phase on.
> 
> This results in an issue because when a posted interrupt is delivered,
> the wakeup_handler() will be executed and fail to find blocked vCPU on
> its per pCPU linked list of all vCPUs that are blocked on this pCPU.
> Which is due to the vCPU being placed on a *different* per pCPU
> linked list than the original pCPU that it had entered block phase.
> 
> The regression is introduced by commit c112b5f50232 ("KVM: x86:
> Recompute PID.ON when clearing PID.SN"). Therefore, partially revert
> it and reintroduce the condition in vmx_vcpu_pi_load() responsible for
> avoiding changing PID.NDST when loading a blocked vCPU.
> 
> Fixes: c112b5f50232 ("KVM: x86: Recompute PID.ON when clearing PID.SN")
> Signed-off-by: Joao Martins <joao.m.martins@...cle.com>
> Signed-off-by: Liran Alon <liran.alon@...cle.com>

Something wrong in the SoB line?

Otherwise looks good.

Paolo

> ---
>  arch/x86/kvm/vmx/vmx.c | 14 ++++++++++++++
>  arch/x86/kvm/vmx/vmx.h |  6 ++++++
>  2 files changed, 20 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 18b0bee662a5..75d903455e1c 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1274,6 +1274,18 @@ static void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int cpu)
>  	if (!pi_test_sn(pi_desc) && vcpu->cpu == cpu)
>  		return;
>  
> +	/*
> +	 * If the 'nv' field is POSTED_INTR_WAKEUP_VECTOR, do not change
> +	 * PI.NDST: pi_post_block is the one expected to change PID.NDST and the
> +	 * wakeup handler expects the vCPU to be on the blocked_vcpu_list that
> +	 * matches PI.NDST. Otherwise, a vcpu may not be able to be woken up
> +	 * correctly.
> +	 */
> +	if (pi_desc->nv == POSTED_INTR_WAKEUP_VECTOR || vcpu->cpu == cpu) {
> +		pi_clear_sn(pi_desc);
> +		goto after_clear_sn;
> +	}
> +
>  	/* The full case.  */
>  	do {
>  		old.control = new.control = pi_desc->control;
> @@ -1289,6 +1301,8 @@ static void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int cpu)
>  	} while (cmpxchg64(&pi_desc->control, old.control,
>  			   new.control) != old.control);
>  
> +after_clear_sn:
> +
>  	/*
>  	 * Clear SN before reading the bitmap.  The VT-d firmware
>  	 * writes the bitmap and reads SN atomically (5.2.3 in the
> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index bee16687dc0b..1e32ab54fc2d 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -373,6 +373,12 @@ static inline void pi_clear_on(struct pi_desc *pi_desc)
>  		(unsigned long *)&pi_desc->control);
>  }
>  
> +static inline void pi_clear_sn(struct pi_desc *pi_desc)
> +{
> +	clear_bit(POSTED_INTR_SN,
> +		(unsigned long *)&pi_desc->control);
> +}
> +
>  static inline int pi_test_on(struct pi_desc *pi_desc)
>  {
>  	return test_bit(POSTED_INTR_ON,
>