Re: [PATCH kernel v3] vfio-pci: Allow mapping MSIX BAR

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 13 Dec 2017 13:31:31 +1100
Alexey Kardashevskiy <aik@ozlabs.ru> wrote:

> By default VFIO disables mapping of MSIX BAR to the userspace as
> the userspace may program it in a way allowing spurious interrupts;
> instead the userspace uses the VFIO_DEVICE_SET_IRQS ioctl.
> In order to eliminate guessing from the userspace about what is
> mmapable, VFIO also advertises a sparse list of regions allowed to mmap.
> 
> This works fine as long as the system page size equals to the MSIX
> alignment requirement which is 4KB. However with a bigger page size
> the existing code prohibits mapping non-MSIX parts of a page with MSIX
> structures so these parts have to be emulated via slow reads/writes on
> a VFIO device fd. If these emulated bits are accessed often, this has
> serious impact on performance.
> 
> This allows mmap of the entire BAR containing MSIX vector table.
> 
> This removes the sparse capability for PCI devices as it becomes useless.
> 
> As the userspace needs to know for sure whether mmapping of the MSIX
> vector containing data can succeed, this adds a new capability -
> VFIO_REGION_INFO_CAP_MSIX_MAPPABLE - which explicitly tells the userspace
> that the entire BAR can be mmapped.
> 
> This does not touch the MSIX mangling in the BAR read/write handlers as
> we are doing this just to enable direct access to non MSIX registers.
> 
> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> ---
> Changes:
> v3:
> * rebased on top of "vfio: Simplify capability helper"
> 
> v2:
> * rewritten vfio_info_add_capability() to provide same interface for
> VFIO_REGION_INFO_CAP_MSIX_MAPPABLE as for other caps.
> ---
>  include/uapi/linux/vfio.h   | 10 +++++++
>  drivers/vfio/pci/vfio_pci.c | 65 +++++++--------------------------------------
>  2 files changed, 19 insertions(+), 56 deletions(-)

Applied to v4.16/vfio.  Thanks,

Alex