Re: [PATCH v2] PCI: s390: Handle ARI on bus without associated struct pci_dev

[Niklas Schnelle <schnelle@linux.ibm.com>]

On Wed, 2024-09-18 at 11:04 +0200, Niklas Schnelle wrote:
> On s390 PCI busses are virtualized and the downstream ports are
> invisible to the OS and struct pci_bus::self is NULL. This associated
> struct pci_dev is however relied upon in pci_ari_enabled() to check
> whether ARI is enabled for the bus. ARI is therefor always detected as
> disabled.
> 
> At the same time firmware on s390 always enables and relies upon ARI
> thus causing a mismatch. Moreover with per-PCI function pass-through
> there may exist busses with no function with devfn 0. For example
> a SR-IOV capable device with two PFs may have separate function
> dependency link chains for each of the PFs and their child VFs. In this
> case the OS may only see the second PF and its child VFs on a bus
> without a devfn 0 function. A situation which is also not supported by
> the common pci_configure_ari() code.
> 
> Dispite simply being a mismatch this causes problems as some PCI devices
> present a different SR-IOV topology depending on PCI_SRIOV_CTRL_ARI.
> 
> A similar mismatch may occur with SR-IOV when virtfn_add_bus() creates new
> busses with no associated struct pci_dev. Here too pci_ari_enabled()
> on these busses would return false even if ARI is actually used.
> 
> Prevent both mismatches by moving the ari_enabled flag from struct
> pci_dev to struct pci_bus making it independent from struct pci_bus::
> self. Let the bus inherit the ari_enabled state from its parent bus when
> there is no bridge device such that busses added by virtfn_add_bus()
> match their parent. For s390 set ari_enabled when the device supports
> ARI in the awareness that all PCIe ports on s390 systems are ARI
> capable.
> 
> Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
> ---
> Note: In a comment of the v1 thread I discussed an alternative idea for
> a generic solution that would include the Jailhouse hypervisor via
> hypervisor_isolated_pci_functions(). As Bjorn correctly pointed out this
> more generic solution however lacks a way to indicate that ARI is really
> enabled in the hardware. So instead for now I propose to stick with this
> patch which only enables this unconditionally on s390x and SR-IOV
> virtual busses where the ARI is inherited.
> ---
> Changes in v2:
> - Rebased on v6.11
> - Link to v1: https://lore.kernel.org/r/20240730-ari_no_bus_dev-v1-1-7de17676f9fe@linux.ibm.com
> 
> 

@Bjorn, this seems to have dropped off your radar or would you like me
to see if I can find a different approach?

Thanks,
Niklas