Re: [RFC PATCH kernel] vfio-pci: Fix sparse capability when no parts of MSIX BAR can be mapped

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

On Thu, 23 Nov 2017 15:56:26 +1100
Alexey Kardashevskiy <aik@ozlabs.ru> wrote:

> It is currently possible to have a sparse capability with 1 areas which
> starts at 0 and 0 bytes long. One example is:
> 
> Texas Instruments TUSB73x0 SuperSpeed USB 3.0 xHCI Host Controller
> [...]
> Region 0: Memory at 3fe280000000 (64-bit, non-prefetchable) [size=64K]
> Region 2: Memory at 3fe280010000 (64-bit, non-prefetchable) [size=8K]
> [...]
> Capabilities: [c0] MSI-X: Enable+ Count=8 Masked-
>         Vector table: BAR=2 offset=00000000
>         PBA: BAR=2 offset=00001000
> 
> With PAGE_SIZE=64K, MSIX BAR occupies the entire BAR2 and cannot be
> mapped.
> 
> This makes it explicit - if sparse->areas is empty, then advertise
> nr_areas as 0.
> 
> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> ---
> 
> QEMU gets it right as vfio_setup_region_sparse_mmaps() checks for size
> after QEMU's 24acf72b9a291ce "vfio: Handle zero-length sparse mmap ranges"
> but why not make it explicit in the first place?
> 
> 
> ---
>  drivers/vfio/pci/vfio_pci.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
> index f041b1a..a201c45 100644
> --- a/drivers/vfio/pci/vfio_pci.c
> +++ b/drivers/vfio/pci/vfio_pci.c
> @@ -597,6 +597,10 @@ static int msix_sparse_mmap_cap(struct vfio_pci_device *vdev,
>  		i++;
>  	}
>  
> +	/* With all alignments, there are no gaps left to mmap */
> +	if (i == 0)
> +		sparse->nr_areas = 0;
> +

Ok, but why does 0 become a special case?  Shouldn't we set
sparse->nr_areas = i?  Thanks,

Alex

>  	ret = vfio_info_add_capability(caps, VFIO_REGION_INFO_CAP_SPARSE_MMAP,
>  				       sparse);
>  	kfree(sparse);