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Re: [RFC PATCH] s390x/pci: vfio-pci breakage with disabled mem enforceme

From: Niklas Schnelle
Subject: Re: [RFC PATCH] s390x/pci: vfio-pci breakage with disabled mem enforcement
Date: Fri, 24 Jul 2020 11:46:39 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.9.0

On 7/23/20 5:13 PM, Matthew Rosato wrote:
> I noticed that after kernel commit abafbc55 'vfio-pci: Invalidate mmaps
> and block MMIO access on disabled memory' vfio-pci via qemu on s390x
> fails spectacularly, with errors in qemu like:
> qemu-system-s390x: vfio_region_read(0001:00:00.0:region0+0x0, 4) failed: 
> Input/output error
> From read to bar 0 originating out of 
> hw/s390x/s390-pci-inst.c:zpci_read_bar().
> So, I'm trying to figure out how to get vfio-pci happy again on s390x.  From
> a bit of tracing, we seem to be triggering the new trap in
> __vfio_pci_memory_enabled().  Sure enough, if I just force this function to
> return 'true' as a test case, things work again.
> The included patch attempts to enforce the setting, which restores everything
> to working order but also triggers vfio_bar_restore() in the process....  So
> this isn't the right answer, more of a proof-of-concept.
> @Alex: Any guidance on what needs to happen to make qemu-s390x happy with this
> recent kernel change?
> @Nilkas/@Pierre: I wonder if this might be related to host device is_virtfn?
> I note that my host device lspci output looks like:
> 0000:00:00.0 Ethernet controller: Mellanox Technologies MT27710 Family 
> [ConnectX-4 Lx Virtual Function]
> But the device is not marked as is_virtfn..  Otherwise, Alex's fix
> from htps://lkml.org/lkml/2020/6/25/628 should cover the case.
With commit e5794cf1a270 ("s390/pci: create links between PFs and VFs") I 
the is_physfn field to struct zpci_dev which gets set through the
CLP Query PCI Function. Also with that commit this being 0 will set
is_virtfn to 1.
Interestingly looking at s390-pci-inst.c in QEMU I'd think that
on QEMU this should already be 0 and thus is_virtfn should be set
with Linux >5.8-rc1 and the missing case is actually for passing through
a PF where it would wrongly be 0 too. 
Note: If the Linux instance does not see the
parent PF however the only way I know to test if it is a VF from userspace
is checking if /sys/bus/pci/devices/<dev>/vfn is non-zero which is platform
specific and currently wrongly set 0 on QEMU for VFs.
If the PF is known the mentioned commit will also create the
/sys/bus/pci/devices/<dev>/physfn symlink as on other platforms.
> Matthew Rosato (1):
>   s390x/pci: Enforce PCI_COMMAND_MEMORY for vfio-pci
>  hw/s390x/s390-pci-inst.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)

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