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

[Alexey Kardashevskiy <aik@ozlabs.ru>]

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;
+
 	ret = vfio_info_add_capability(caps, VFIO_REGION_INFO_CAP_SPARSE_MMAP,
 				       sparse);
 	kfree(sparse);
-- 
2.11.0