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It's my view now that we likely > > went too far with 75cc460a1b, and Xen disabling memory decoding of > > devices (as buggy as they might be) is harmful, and reduces the set of > > hardware on which Xen works. > > > > This commits reverts most of 75cc460a1b, and instead adds checks to > > vPCI in order to prevent misplaced BARs from being added to the > > hardware domain p2m. > > Which makes me wonder: How do things work then? Dom0 then still can't > access the BAR address range, can it? It does allow access on some situations where the previous arrangement didn't work because it wholesale disabled memory decoding for the device. So if it's only one BAR that's misplaced the rest will still get added to the dom0 p2m and be accessible, because memory decoding won't be turned off for the device. > Plus with this adjustment, is > ... > > > Signaling on whether BARs are mapped is tracked > > in the vpci structure, so that misplaced BARs are not mapped, and thus > > Xen won't attempt to unmap them when memory decoding is disabled. > > > > This restores the behavior of Xen for PV dom0 to the state it was > > previous to 75cc460a1b, while also introducing a more contained fix > > for the vPCI BAR mapping issues. > > ... this (in particular "restores the behavior") a valid description > of this change? Yes, it restores the previous behavior for PV dom0, as memory decoding is no longer turned off for any devices regardless of where the BARs are positioned. > > Fixes: 75cc460a1b ('xen/pci: detect when BARs are not suitably positioned') > > Signed-off-by: Roger Pau Monné > > --- > > AT Citrix we have a system with a device with the following BARs: > > > > BAR [0xfe010, 0xfe010] -> in a EfiMemoryMappedIO region > > BAR [0, 0x1fff] -> not positioned, outside host bridge window > > > > And memory decoding enabled by the firmware. With the current code > > (or any of the previous fix proposals), Xen would still disable memory > > decoding for the device, and the system will freeze when attempting to > > set EFI vars. > > Isn't the latter (BAR at address 0) yet another problem? It's a BAR that hasn't been positioned by the firmware AFAICT. Which is a bug in the firmware but shouldn't prevent Xen from booting. In the above system address 0 is outside of the PCI host bridge window, so even if we mapped the BAR and memory decoding for the device was enabled accessing such BAR wouldn't work. > I have to admit > that I'm uncertain in how far it is a good idea to try to make Xen look > to work on such a system ... PV dom0 works on a system like the above prior to c/s 75cc460a1b, so I would consider 75cc460a1b to be a regression for PV dom0 setups. Thanks, Roger.