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

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Message-ID: <20200909170746.2286b83a@w520.home>
Date:   Wed, 9 Sep 2020 17:07:46 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Matthew Rosato <mjrosato@...ux.ibm.com>
Cc:     Bjorn Helgaas <helgaas@...nel.org>, 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, 3 Sep 2020 13:10:02 -0400
Matthew Rosato <mjrosato@...ux.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@...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
> >>  
>