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

[Alex Williamson]

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.  PCI BAR sizing through the
BAR register still needs to work via the kernel interface alone.  It's
clear now how the kernel resizing the vma on mmap was a hack around
userspace mangling the region size.

Maybe this needs to be exposed as a device specific region, which then
userspace emulates as a BAR for the VM facing device rather than the
kernel emulating it as a BAR.  Thanks,

Alex