Re: [PATCH] PCI: Introduce flag for detached virtual functions

From: Matthew Rosato
Date: Wed Aug 12 2020 - 14:04:14 EST

On 8/12/20 12:43 PM, Alex Williamson wrote:
On Wed, 12 Aug 2020 10:50:17 -0400
Matthew Rosato <mjrosato@xxxxxxxxxxxxx> wrote:

s390x has the notion of providing VFs to the kernel in a manner
where the associated PF is inaccessible other than via firmware.
These are not treated as typical VFs and access to them is emulated
by underlying firmware which can still access the PF. After
abafbc55 however these detached VFs were no longer able to work
with vfio-pci as the firmware does not provide emulation of the
PCI_COMMAND_MEMORY bit. In this case, let's explicitly recognize
these detached VFs so that vfio-pci can allow memory access to
them again.

Signed-off-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
Reviewed-by: Pierre Morel <pmorel@xxxxxxxxxxxxx>
Reviewed-by: Niklas Schnelle <schnelle@xxxxxxxxxxxxx>
arch/s390/pci/pci.c | 8 ++++++++
drivers/vfio/pci/vfio_pci_config.c | 3 ++-
include/linux/pci.h | 1 +
3 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/s390/pci/pci.c b/arch/s390/pci/pci.c
index 3902c9f..04ac76d 100644
--- a/arch/s390/pci/pci.c
+++ b/arch/s390/pci/pci.c
@@ -581,6 +581,14 @@ int pcibios_enable_device(struct pci_dev *pdev, int mask)
struct zpci_dev *zdev = to_zpci(pdev);
+ /*
+ * If we have a VF on a non-multifunction bus, it must be a VF that is
+ * detached from its parent PF. We rely on firmware emulation to
+ * provide underlying PF details.
+ */
+ if (zdev->vfn && !zdev->zbus->multifunction)
+ pdev->detached_vf = 1;
zpci_debug_init_device(zdev, dev_name(&pdev->dev));
diff --git a/drivers/vfio/pci/vfio_pci_config.c b/drivers/vfio/pci/vfio_pci_config.c
index d98843f..17845fc 100644
--- a/drivers/vfio/pci/vfio_pci_config.c
+++ b/drivers/vfio/pci/vfio_pci_config.c
@@ -406,7 +406,8 @@ bool __vfio_pci_memory_enabled(struct vfio_pci_device *vdev)
* PF SR-IOV capability, there's therefore no need to trigger
* faults based on the virtual value.
- return pdev->is_virtfn || (cmd & PCI_COMMAND_MEMORY);
+ return pdev->is_virtfn || pdev->detached_vf ||

Wouldn't we also want to enable the is_virtfn related code in
vfio_basic_config_read() and at least the initial setting of the
command register in vfio_config_init()? Otherwise we're extending the
incomplete emulation out to userspace. Thanks,

We had discussed doing this internally and I ultimately left it out of this patch to keep the changes small... But I agree that it makes sense to fix the emulation for userspace. I can add the changes you suggest + would also need to change vfio_restore_bar() with:

- if (pdev->is_virtfn)
+ if (pdev->is_virtfn || pdev->detached_vf)

to prevent the config changes from tripping the restore code.

I'll send a V2 with this included.