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

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Date:   Thu, 3 Sep 2020 11:41:17 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Matthew Rosato <mjrosato@...ux.ibm.com>
Cc:     alex.williamson@...hat.com, bhelgaas@...gle.com,
        schnelle@...ux.ibm.com, pmorel@...ux.ibm.com, mpe@...erman.id.au,
        oohall@...il.com, cohuck@...hat.com, kevin.tian@...el.com,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH v4 1/3] PCI/IOV: Mark VFs as not implementing MSE bit

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@...ux.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
>