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

Bjorn Helgaas
2020-09-03T16:41:22Z

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.

- "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".

- 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.

- Wrap the commit log to fill a 75-char line. It's arbitrary, but
that's what I use for consistency.

> 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
>