Re: [PATCH kvmtool v1 2/2] vfio-pci: Fallback to INTx mode when disable MSI/MSIX

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On 15/03/2019 08:33, Leo Yan wrote:
> Since PCI forbids enabling INTx, MSI or MSIX at the same time, it's by
> default to disable INTx mode when enable MSI/MSIX mode; but this logic is
> easily broken if the guest PCI driver detects the MSI/MSIX cannot work as
> expected and tries to rollback to use INTx mode.  The INTx mode has been
> disabled and it has no chance to be enabled again, thus both INTx mode
> and MSI/MSIX mode will not be enabled in vfio for this case.
> 
> Below shows the detailed flow for introducing this issue:
> 
>   vfio_pci_configure_dev_irqs()
>     `-> vfio_pci_enable_intx()
> 
>   vfio_pci_enable_msis()
>     `-> vfio_pci_disable_intx()
> 
>   vfio_pci_disable_msis()   => Guest PCI driver disables MSI
> 
> To fix this issue, when disable MSI/MSIX we need to check if INTx mode
> is available for this device or not; if the device can support INTx then
> we need to re-enable it so the device can fallback to use it.
> 
> Signed-off-by: Leo Yan <leo.yan@xxxxxxxxxx>
> ---
>  vfio/pci.c | 17 ++++++++++++-----
>  1 file changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/vfio/pci.c b/vfio/pci.c
> index c0683f6..44727bb 100644
> --- a/vfio/pci.c
> +++ b/vfio/pci.c
> @@ -28,6 +28,7 @@ struct vfio_irq_eventfd {
>  	msi_update_state(state, val, VFIO_PCI_MSI_STATE_EMPTY)
>  
>  static void vfio_pci_disable_intx(struct kvm *kvm, struct vfio_device *vdev);
> +static int vfio_pci_enable_intx(struct kvm *kvm, struct vfio_device *vdev);
>  
>  static int vfio_pci_enable_msis(struct kvm *kvm, struct vfio_device *vdev,
>  				bool msix)
> @@ -50,7 +51,7 @@ static int vfio_pci_enable_msis(struct kvm *kvm, struct vfio_device *vdev,
>  	if (!msi_is_enabled(msis->virt_state))
>  		return 0;
>  
> -	if (pdev->irq_modes & VFIO_PCI_IRQ_MODE_INTX) {
> +	if (pdev->irq_modes & VFIO_PCI_IRQ_MODE_INTX)
>  		/*
>  		 * PCI (and VFIO) forbids enabling INTx, MSI or MSIX at the same
>  		 * time. Since INTx has to be enabled from the start (we don't
> @@ -58,9 +59,6 @@ static int vfio_pci_enable_msis(struct kvm *kvm, struct vfio_device *vdev,
>  		 * disable it now.
>  		 */
>  		vfio_pci_disable_intx(kvm, vdev);
> -		/* Permanently disable INTx */
> -		pdev->irq_modes &= ~VFIO_PCI_IRQ_MODE_INTX;

As a result vfio_pci_disable_intx() may be called multiple times (each
time the guest enables one MSI vector). Could you make
vfio_pci_disable_intx() safe against that (maybe use intx_fd == -1 to
denote the INTx state)?

> -	}
>  
>  	eventfds = (void *)msis->irq_set + sizeof(struct vfio_irq_set);
>  
> @@ -162,7 +160,16 @@ static int vfio_pci_disable_msis(struct kvm *kvm, struct vfio_device *vdev,
>  	msi_set_enabled(msis->phys_state, false);
>  	msi_set_empty(msis->phys_state, true);
>  
> -	return 0;
> +	if (pdev->irq_modes & VFIO_PCI_IRQ_MODE_INTX)
> +		/*
> +		 * When MSI or MSIX is disabled, this might be called when
> +		 * PCI driver detects the MSI interrupt failure and wants to
> +		 * rollback to INTx mode.  Thus enable INTx if the device
> +		 * supports INTx mode in this case.
> +		 */
> +		ret = vfio_pci_enable_intx(kvm, vdev);

Let's remove vfio_pci_reserve_irq_fds(2) from vfio_pci_enable_intx(), it
should only called once per run, and isn't particularly useful here
since INTx only uses 2 fds. It's used to bump the fd rlimit when a
device needs ~2048 file descriptors for MSI-X.

Thanks,
Jean

> +
> +	return ret >= 0 ? 0 : ret;
>  }
>  
>  static int vfio_pci_update_msi_entry(struct kvm *kvm, struct vfio_device *vdev,
> 

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