PCI: Expose PCIe Resizable BAR support via sysfs

Add a simple sysfs interface to Resizable BAR support, largely for the
purposes of assigning such devices to a VM through VFIO.  Resizable BARs
present a difficult feature to expose to a VM through emulation, as
resizing a BAR is done on the host.  It can fail, and often does, but we
have no means via emulation of a PCIe REBAR capability to handle the error
cases.

A vfio-pci specific ioctl interface is also cumbersome as there are often
multiple devices within the same bridge aperture and handling them is a
challenge.  In the interface proposed here, expanding a BAR potentially
requires such devices to be soft-removed during the resize operation and
rescanned after, in order for all the necessary resources to be released.
A pci-sysfs interface is also more universal than a vfio specific
interface.

Please see the ABI documentation update for usage.

Link: https://lore.kernel.org/r/166336088796.3597940.14973499936692558556.stgit@omen
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Cc: Krzysztof Wilczyński <kw@linux.com>

1 files changed, 33 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-bus-pci b/Documentation/ABI/testing/sysfs-bus-pci
index 6fc2c2efe8ab..840727fc75dc 100644
--- a/Documentation/ABI/testing/sysfs-bus-pci
+++ b/Documentation/ABI/testing/sysfs-bus-pci
@@ -457,3 +457,36 @@ Description:
 
 		The file is writable if the PF is bound to a driver that
 		implements ->sriov_set_msix_vec_count().
+
+What:		/sys/bus/pci/devices/.../resourceN_resize
+Date:		September 2022
+Contact:	Alex Williamson <alex.williamson@redhat.com>
+Description:
+		These files provide an interface to PCIe Resizable BAR support.
+		A file is created for each BAR resource (N) supported by the
+		PCIe Resizable BAR extended capability of the device.  Reading
+		each file exposes the bitmap of available resource sizes:
+
+		# cat resource1_resize
+		00000000000001c0
+
+		The bitmap represents supported resource sizes for the BAR,
+		where bit0 = 1MB, bit1 = 2MB, bit2 = 4MB, etc.  In the above
+		example the device supports 64MB, 128MB, and 256MB BAR sizes.
+
+		When writing the file, the user provides the bit position of
+		the desired resource size, for example:
+
+		# echo 7 > resource1_resize
+
+		This indicates to set the size value corresponding to bit 7,
+		128MB.  The resulting size is 2 ^ (bit# + 20).  This definition
+		matches the PCIe specification of this capability.
+
+		In order to make use of resource resizing, all PCI drivers must
+		be unbound from the device and peer devices under the same
+		parent bridge may need to be soft removed.  In the case of
+		VGA devices, writing a resize value will remove low level
+		console drivers from the device.  Raw users of pci-sysfs
+		resourceN attributes must be terminated prior to resizing.
+		Success of the resizing operation is not guaranteed.