Re: [Qemu-devel] [PATCH] vfio: allow to disable MMAP per device with -x-mmap=off option

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

On Tue, 2015-02-24 at 14:40 -0700, Eric Blake wrote:
> On 02/24/2015 01:38 PM, Samuel Pitoiset wrote:
> > Disabling MMAP support uses the slower read/write accesses but allows to
> > trace all MMIO accesses, which is not good for performance, but very
> > useful for reverse engineering PCI drivers. This option allows to
> > disable MMAP per device without a compile-time change.
> > 
> > Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
> > ---
> >  hw/vfio/common.c              | 2 +-
> >  hw/vfio/pci.c                 | 1 +
> >  include/hw/vfio/vfio-common.h | 2 +-
> >  3 files changed, 3 insertions(+), 2 deletions(-)
> 
> > +++ b/hw/vfio/pci.c
> > @@ -3456,6 +3456,7 @@ static Property vfio_pci_dev_properties[] = {
> >      DEFINE_PROP_BIT("x-vga", VFIOPCIDevice, features,
> >                      VFIO_FEATURE_ENABLE_VGA_BIT, false),
> >      DEFINE_PROP_INT32("bootindex", VFIOPCIDevice, bootindex, -1),
> > +    DEFINE_PROP_BOOL("x-mmap", VFIOPCIDevice, vbasedev.allow_mmap, true),
> 
> Naming it 'x-mmap' implies it is experimental and may be removed
> someday.  Is there any reason why you are not proposing it as a
> permanent knob?

I actually suggested the x- prefix to Samuel because I don't think this
is a mode that we want to officially support.  It's useful for enabling
tracing, but it's possible that doing this will break timing sensitive
devices.  I therefore don't really want to put it at the fingertips of
the average user.  If we have a reason to call it supported at some
point later, we can always change it.  Thanks,

Alex