Re: [RFC v1 4/4] qemu: adjust queried bar size to power-of-2

[Philippe Mathieu-Daudé]

On 6/6/23 14:54, Alex Williamson wrote:
> On Mon, 5 Jun 2023 16:50:05 -0700
> <ankita@nvidia.com> wrote:
>
> > From: Ankit Agrawal <ankita@nvidia.com>
> >
> > The GPU device memory is reported to the VM as a BAR. The device memory
> > may not be aligned to the power-of-2, but the QEMU expects the PCI BAR to
> > be. Align the reported device memory size to the next power-of-2 before
> > QEMU does an mmap.
> >
> > Signed-off-by: Ankit Agrawal <ankita@nvidia.com>
> > ---
> >   hw/vfio/common.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/hw/vfio/common.c b/hw/vfio/common.c
> > index 4d01ea3515..bb49200458 100644
> > --- a/hw/vfio/common.c
> > +++ b/hw/vfio/common.c
> > @@ -2061,7 +2061,7 @@ int vfio_region_setup(Object *obj, VFIODevice *vbasedev, 
> > VFIORegion *region,
> >   
> >       region->vbasedev = vbasedev;
> >       region->flags = info->flags;
> > -    region->size = info->size;
> > +    region->size = info->size ? pow2ceil(info->size) : info->size;
> >       region->fd_offset = info->offset;
> >       region->nr = index;
> >   
>
> Nak, this means our kernel emulation of the BAR is broken, a BAR that
> is not naturally aligned is not a PCI BAR.

Right. So the common code could check this value is correct, like:

  assert(is_power_of_2(->size));

Or less violet using error_report :)