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

Matthew Rosato
2020-09-03T17:10:26Z

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.

>
>> 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
>>