Re: [PATCH 4/4] KVM: x86: Use a stable condition around all VT-d PI paths

[David Matlack <dmatlack@google.com>]

On Mon, Nov 22, 2021 at 07:43:11PM -0500, Paolo Bonzini wrote:
> Currently, checks for whether VT-d PI can be used refer to the current
> status of the feature in the current vCPU; or they more or less pick
> vCPU 0 in case a specific vCPU is not available.
> 
> However, these checks do not attempt to synchronize with changes to
> the IRTE.  In particular, there is no path that updates the IRTE when
> APICv is re-activated on vCPU 0; and there is no path to wakeup a CPU
> that has APICv disabled, if the wakeup occurs because of an IRTE
> that points to a posted interrupt.
> 
> To fix this, always go through the VT-d PI path as long as there are
> assigned devices and APICv is available on both the host and the VM side.
> Since the relevant condition was copied over three times, take the hint
> and factor it into a separate function.
> 
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Cc: stable@vger.kernel.org
> Reviewed-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Reviewed-by: David Matlack <dmatlack@google.com>

> ---
>  arch/x86/kvm/vmx/posted_intr.c | 20 +++++++++++---------
>  1 file changed, 11 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/posted_intr.c b/arch/x86/kvm/vmx/posted_intr.c
> index 5f81ef092bd4..1c94783b5a54 100644
> --- a/arch/x86/kvm/vmx/posted_intr.c
> +++ b/arch/x86/kvm/vmx/posted_intr.c
> @@ -5,6 +5,7 @@
>  #include <asm/cpu.h>
>  
>  #include "lapic.h"
> +#include "irq.h"
>  #include "posted_intr.h"
>  #include "trace.h"
>  #include "vmx.h"
> @@ -77,13 +78,18 @@ void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int cpu)
>  		pi_set_on(pi_desc);
>  }
>  
> +static bool vmx_can_use_vtd_pi(struct kvm *kvm)
> +{
> +	return irqchip_in_kernel(kvm) && enable_apicv &&
> +		kvm_arch_has_assigned_device(kvm) &&
> +		irq_remapping_cap(IRQ_POSTING_CAP);
> +}
> +
>  void vmx_vcpu_pi_put(struct kvm_vcpu *vcpu)
>  {
>  	struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);
>  
> -	if (!kvm_arch_has_assigned_device(vcpu->kvm) ||
> -		!irq_remapping_cap(IRQ_POSTING_CAP)  ||
> -		!kvm_vcpu_apicv_active(vcpu))
> +	if (!vmx_can_use_vtd_pi(vcpu->kvm))
>  		return;
>  
>  	/* Set SN when the vCPU is preempted */
> @@ -141,9 +147,7 @@ int pi_pre_block(struct kvm_vcpu *vcpu)
>  	struct pi_desc old, new;
>  	struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);
>  
> -	if (!kvm_arch_has_assigned_device(vcpu->kvm) ||
> -		!irq_remapping_cap(IRQ_POSTING_CAP)  ||
> -		!kvm_vcpu_apicv_active(vcpu))
> +	if (!vmx_can_use_vtd_pi(vcpu->kvm))
>  		return 0;
>  
>  	WARN_ON(irqs_disabled());
> @@ -270,9 +274,7 @@ int pi_update_irte(struct kvm *kvm, unsigned int host_irq, uint32_t guest_irq,
>  	struct vcpu_data vcpu_info;
>  	int idx, ret = 0;
>  
> -	if (!kvm_arch_has_assigned_device(kvm) ||
> -	    !irq_remapping_cap(IRQ_POSTING_CAP) ||
> -	    !kvm_vcpu_apicv_active(kvm->vcpus[0]))
> +	if (!vmx_can_use_vtd_pi(kvm))
>  		return 0;
>  
>  	idx = srcu_read_lock(&kvm->irq_srcu);
> -- 
> 2.27.0
>