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

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Message-ID: <20200910003916.GA741660@bjorn-Precision-5520>
Date:   Wed, 9 Sep 2020 19:39:16 -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 Thu, Sep 03, 2020 at 01:10:02PM -0400, Matthew Rosato wrote:
> On 9/3/20 12:41 PM, Bjorn Helgaas wrote:

> >    - 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),

Sorry, I wasn't clear about this.  I was trying to suggest that if
there are some situations where we need to use pci_dev_flags instead
of a bitfield, it would be useful to have a generic comment to help
decide between them.

I don't know that there *is* a good reason, and unless somebody can
think of one, I'd like to get rid of pci_dev_flags completely and
convert them all to bitfields.

Given that, my preference would be to just add a new bitfield,
something like this:

  struct pci_dev {
    ...
    unsigned int no_command_memory:1;  /* No PCI_COMMAND_MEMORY */