qemu/hw/vfio
Alex Williamson b5048a4cbf vfio/pci: Static Resizable BAR capability
The PCI Resizable BAR (ReBAR) capability is currently hidden from the
VM because the protocol for interacting with the capability does not
support a mechanism for the device to reject an advertised supported
BAR size.  However, when assigned to a VM, the act of resizing the
BAR requires adjustment of host resources for the device, which
absolutely can fail.  Linux does not currently allow us to reserve
resources for the device independent of the current usage.

The only writable field within the ReBAR capability is the BAR Size
register.  The PCIe spec indicates that when written, the device
should immediately begin to operate with the provided BAR size.  The
spec however also notes that software must only write values
corresponding to supported sizes as indicated in the capability and
control registers.  Writing unsupported sizes produces undefined
results.  Therefore, if the hypervisor were to virtualize the
capability and control registers such that the current size is the
only indicated available size, then a write of anything other than
the current size falls into the category of undefined behavior,
where we can essentially expose the modified ReBAR capability as
read-only.

This may seem pointless, but users have reported that virtualizing
the capability in this way not only allows guest software to expose
related features as available (even if only cosmetic), but in some
scenarios can resolve guest driver issues.  Additionally, no
regressions in behavior have been reported for this change.

A caveat here is that the PCIe spec requires for compatibility that
devices report support for a size in the range of 1MB to 512GB,
therefore if the current BAR size falls outside that range we revert
to hiding the capability.

Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/r/20230505232308.2869912-1-alex.williamson@redhat.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
2023-05-09 09:30:13 -06:00
..
amd-xgbe.c Include migration/vmstate.h less 2019-08-16 13:31:52 +02:00
ap.c target/s390x: move kvm files into kvm/ 2021-07-07 14:01:59 +02:00
calxeda-xgmac.c Include migration/vmstate.h less 2019-08-16 13:31:52 +02:00
ccw.c hw/vfio/ccw: Replace DO_UPCAST(VFIOCCWDevice) by VFIO_CCW() 2023-02-27 09:15:38 +01:00
common.c vfio/migration: Skip log_sync during migration SETUP state 2023-05-09 09:30:13 -06:00
display.c *: Add missing includes of qemu/error-report.h 2023-03-22 15:06:57 +00:00
igd.c *: Add missing includes of qemu/error-report.h 2023-03-22 15:06:57 +00:00
Kconfig hw/vfio: Move the IGD quirk code to a separate file 2020-02-06 11:55:42 -07:00
meson.build vfio: Add migration region initialization and finalize function 2020-11-01 12:30:50 -07:00
migration.c *: Add missing includes of qemu/error-report.h 2023-03-22 15:06:57 +00:00
pci-quirks.c hw/vfio/pci-quirks: Resolve redundant property getters 2022-05-24 10:38:50 +10:00
pci.c vfio/pci: Static Resizable BAR capability 2023-05-09 09:30:13 -06:00
pci.h vfio/pci: add support for VF token 2023-05-09 09:30:13 -06:00
platform.c Use g_new() & friends where that makes obvious sense 2022-03-21 15:44:44 +01:00
spapr.c Replace qemu_real_host_page variables with inlined functions 2022-04-06 10:50:38 +02:00
trace-events vfio: Fix vfio_get_dev_region() trace event 2023-03-07 11:19:07 -07:00
trace.h trace: switch position of headers to what Meson requires 2020-08-21 06:18:24 -04:00