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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> |
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| .. | ||
| amd-xgbe.c | ||
| ap.c | ||
| calxeda-xgmac.c | ||
| ccw.c | ||
| common.c | ||
| display.c | ||
| igd.c | ||
| Kconfig | ||
| meson.build | ||
| migration.c | ||
| pci-quirks.c | ||
| pci.c | ||
| pci.h | ||
| platform.c | ||
| spapr.c | ||
| trace-events | ||
| trace.h | ||