Re: [PATCH v4 1/3] PCI/IOV: Mark VFs as not implementing MSE bit

Alex Williamson
2020-09-10T02:24:47Z

On Thu, 3 Sep 2020 13:10:02 -0400
Matthew Rosato <mjrosato@linux.ibm.com> wrote:

> On 9/3/20 12:41 PM, Bjorn Helgaas wrote:
> > On Wed, Sep 02, 2020 at 03:46:34PM -0400, Matthew Rosato wrote:
> >> Per the PCIe spec, VFs cannot implement the MSE bit
> >> AKA PCI_COMMAND_MEMORY, and it must be hard-wired to 0.
> >> Use a dev_flags bit to signify this requirement.
> >
> > This approach seems sensible to me, but
> >
> > - This is confusing because while the spec does not use "MSE" to
> > refer to the Command Register "Memory Space Enable" bit
> > (PCI_COMMAND_MEMORY), it *does* use "MSE" in the context of the
> > "VF MSE" bit, which is in the PF SR-IOV Capability. But of
> > course, you're not talking about that here. Maybe something like
> > this?
> >
> > For VFs, the Memory Space Enable bit in the Command Register is
> > hard-wired to 0.
> >
> > Add a dev_flags bit to signify devices where the Command
> > Register Memory Space Enable bit does not control the device's
> > response to MMIO accesses.
>
> Will do. I'll change the usage of the MSE acronym in the other patches
> as well.
>
> >
> > - "PCI_DEV_FLAGS_FORCE_COMMAND_MEM" says something about how you
> > plan to *use* this, but I'd rather use a term that describes the
> > hardware, e.g., "PCI_DEV_FLAGS_NO_COMMAND_MEMORY".
>
> Sure, I will change.
>
> >
> > - How do we decide whether to use dev_flags vs a bitfield like
> > dev->is_virtfn? The latter seems simpler unless there's a reason
> > to use dev_flags. If there's a reason, maybe we could add a
> > comment at pci_dev_flags for future reference.
> >
>
> Something like:
>
> /*
> * Device does not implement PCI_COMMAND_MEMORY - this is true for any
> * device marked is_virtfn, but is also true for any VF passed-through
> * a lower-level hypervisor where emulation of the Memory Space Enable
> * bit was not provided.
> */
> PCI_DEV_FLAGS_NO_COMMAND_MEMORY = (__force pci_dev_flags_t) (1 << 12),
>
> ?
>
> > - Wrap the commit log to fill a 75-char line. It's arbitrary, but
> > that's what I use for consistency.
>
> Sure, will do. I'll roll up a new version once I have feedback from
> Alex on the vfio changes.

The usage of MSE threw me a bit too, as Bjorn notes that's specific to
the SR-IOV capability. I think this also uncovers a latent bug in our
calling of vfio_bar_restore(), it really doesn't do a good job of
determining whether an enable bit is implemented, regardless of whether
it's a VF or the device simply doesn't use that address space. For
example I imagine you could reproduce triggering a reset recovery on
s390 by trying to write the VF command register to 1 with setpci from a
guest (since you won't have is_virtfn to bail out of the recovery
function). I think we'll still need this dev_flag to differentiate
unimplmented and enabled versus simply unimplemented to resolve that
though, so the change looks ok to me. Thanks,

Alex

> >> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
> >> ---
> >> drivers/pci/iov.c | 1 +
> >> include/linux/pci.h | 2 ++
> >> 2 files changed, 3 insertions(+)
> >>
> >> diff --git a/drivers/pci/iov.c b/drivers/pci/iov.c
> >> index b37e08c..2bec77c 100644
> >> --- a/drivers/pci/iov.c
> >> +++ b/drivers/pci/iov.c
> >> @@ -180,6 +180,7 @@ int pci_iov_add_virtfn(struct pci_dev *dev, int id)
> >> virtfn->device = iov->vf_device;
> >> virtfn->is_virtfn = 1;
> >> virtfn->physfn = pci_dev_get(dev);
> >> + virtfn->dev_flags |= PCI_DEV_FLAGS_FORCE_COMMAND_MEM;
> >>
> >> if (id == 0)
> >> pci_read_vf_config_common(virtfn);
> >> diff --git a/include/linux/pci.h b/include/linux/pci.h
> >> index 8355306..9316cce 100644
> >> --- a/include/linux/pci.h
> >> +++ b/include/linux/pci.h
> >> @@ -227,6 +227,8 @@ enum pci_dev_flags {
> >> PCI_DEV_FLAGS_NO_FLR_RESET = (__force pci_dev_flags_t) (1 << 10),
> >> /* Don't use Relaxed Ordering for TLPs directed at this device */
> >> PCI_DEV_FLAGS_NO_RELAXED_ORDERING = (__force pci_dev_flags_t) (1 << 11),
> >> + /* Device does not implement PCI_COMMAND_MEMORY (e.g. a VF) */
> >> + PCI_DEV_FLAGS_FORCE_COMMAND_MEM = (__force pci_dev_flags_t) (1 << 12),
> >> };
> >>
> >> enum pci_irq_reroute_variant {
> >> --
> >> 1.8.3.1
> >>
>