Subject: Re: [RFC PATCH v2 3/3] vfio-pci: Allow to mmap MSI-X table if EEH is supported

From: Alex Williamson <[email-masked]>
Date: Mon, 04 Jan 2016 14:07:35 -0700

On Thu, 2015-12-31 at 16:50 +0800, Yongji Xie wrote:
> Current vfio-pci implementation disallows to mmap MSI-X
> table in case that user get to touch this directly.
> 
> However, EEH mechanism can ensure that a given pci device
> can only shoot the MSIs assigned for its PE. So we think
> it's safe to expose the MSI-X table to userspace because
> the exposed MSI-X table can't be used to do harm to other
> memory space.
> 
> And with MSI-X table mmapped, some performance issues which
> are caused when PCI adapters have critical registers in the
> same page as the MSI-X table also can be resolved.
> 
> So this patch adds a Kconfig option, VFIO_PCI_MMAP_MSIX,
> to support for mmapping MSI-X table.
> 
> Signed-off-by: Yongji Xie <[email-masked]>
> ---
>  drivers/vfio/pci/Kconfig    |    4 ++++
>  drivers/vfio/pci/vfio_pci.c |    6 ++++--
>  2 files changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/vfio/pci/Kconfig b/drivers/vfio/pci/Kconfig
> index 02912f1..67b0a2c 100644
> --- a/drivers/vfio/pci/Kconfig
> +++ b/drivers/vfio/pci/Kconfig
> @@ -23,6 +23,10 @@ config VFIO_PCI_MMAP
>  	depends on VFIO_PCI
>  	def_bool y if !S390
>  
> +config VFIO_PCI_MMAP_MSIX
> +	depends on VFIO_PCI_MMAP
> +	def_bool y if EEH

Does CONFIG_EEH necessarily mean the EEH is enabled?  Could the system
not support EEH or could EEH be disabled via kernel commandline
options?

> +
>  config VFIO_PCI_INTX
>  	depends on VFIO_PCI
>  	def_bool y if !S390
> diff --git a/drivers/vfio/pci/vfio_pci.c
> b/drivers/vfio/pci/vfio_pci.c
> index 09b3805..d536985 100644
> --- a/drivers/vfio/pci/vfio_pci.c
> +++ b/drivers/vfio/pci/vfio_pci.c
> @@ -555,7 +555,8 @@ static long vfio_pci_ioctl(void *device_data,
>  			    IORESOURCE_MEM && (info.size >=
> PAGE_SIZE ||
>  			    pci_resource_page_aligned)) {
>  				info.flags |=
> VFIO_REGION_INFO_FLAG_MMAP;
> -				if (info.index == vdev->msix_bar) {
> +				if
> (!IS_ENABLED(CONFIG_VFIO_PCI_MMAP_MSIX) &&
> +				    info.index == vdev->msix_bar) {
>  					ret =
> msix_sparse_mmap_cap(vdev, &caps);
>  					if (ret)
>  						return ret;
> @@ -967,7 +968,8 @@ static int vfio_pci_mmap(void *device_data,
> struct vm_area_struct *vma)
>  	if (phys_len < PAGE_SIZE || req_start + req_len > phys_len)
>  		return -EINVAL;
>  
> -	if (index == vdev->msix_bar) {
> +	if (!IS_ENABLED(CONFIG_VFIO_PCI_MMAP_MSIX) &&
> +	    index == vdev->msix_bar) {
>  		/*
>  		 * Disallow mmaps overlapping the MSI-X table; users
> don't
>  		 * get to touch this directly.  We could find
> somewhere