linux-kernel - Re: [PATCH V3] KVM: x86: Sync the pending Posted-Interrupts

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Date:   Thu, 31 Jan 2019 10:25:24 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Luwei Kang <luwei.kang@...el.com>, rkrcmar@...hat.com,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        x86@...nel.org
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3] KVM: x86: Sync the pending Posted-Interrupts

On 31/01/19 09:52, Luwei Kang wrote:
> Some Posted-Interrupts from passthrough devices may be lost or
> overwritten when the vCPU is in runnable state.
> 
> The SN (Suppress Notification) of PID (Posted Interrupt Descriptor) will
> be set when the vCPU is preempted (vCPU in KVM_MP_STATE_RUNNABLE state
> but not running on physical CPU). If a posted interrupt coming at this
> time, the irq remmaping facility will set the bit of PIR (Posted
> Interrupt Requests) without ON (Outstanding Notification).
> So this interrupt can't be sync to APIC virtualization register and
> will not be handled by Guest because ON is zero.
> 
> Signed-off-by: Luwei Kang <luwei.kang@...el.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 5 +++++
>  arch/x86/kvm/x86.c     | 2 +-
>  2 files changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 4341175..8ed9634 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1221,6 +1221,11 @@ static void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int cpu)
>  		new.sn = 0;
>  	} while (cmpxchg64(&pi_desc->control, old.control,
>  			   new.control) != old.control);
> +

	/*
	 * Clear SN before reading the bitmap.  The VT-d firmware
	 * writes the bitmap and reads SN atomically (5.2.3 in the
	 * spec), so it doesn't really have a memory barrier that
	 * pairs with this, but we cannot do that and we need one.
	 */

> +	smp_mb__after_atomic();
> +
> +	if (!bitmap_empty((unsigned long *)pi_desc->pir, NR_VECTORS))
> +		pi_test_and_set_on(pi_desc);

You can add pi_set_on for use here.  The fast path with pi_clear_sn
should also be removed.

>  }

>  /*
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 3d27206..5bcf2c4 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7794,7 +7794,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  	 * 1) We should set ->mode before checking ->requests.  Please see
>  	 * the comment in kvm_vcpu_exiting_guest_mode().
>  	 *
> -	 * 2) For APICv, we should set ->mode before checking PIR.ON.  This
> +	 * 2) For APICv, we should set ->mode before checking PID.PIR. This

This should be PID.ON.

Paolo

>  	 * pairs with the memory barrier implicit in pi_test_and_set_on
>  	 * (see vmx_deliver_posted_interrupt).
>  	 *
>