Re: [PATCH v2] PCI: Introduce flag for detached virtual functions

[Matthew Rosato]

On 8/12/20 4:32 PM, Alex Williamson wrote:
> On Wed, 12 Aug 2020 15:21:11 -0400
> Matthew Rosato <mjrosato@xxxxxxxxxxxxx> wrote:
>
> > s390x has the notion of providing VFs to the kernel in a manner
> > where the associated PF is inaccessible other than via firmware.
> > These are not treated as typical VFs and access to them is emulated
> > by underlying firmware which can still access the PF.  After
> > abafbc55 however these detached VFs were no longer able to work
> > with vfio-pci as the firmware does not provide emulation of the
> > PCI_COMMAND_MEMORY bit.  In this case, let's explicitly recognize
> > these detached VFs so that vfio-pci can allow memory access to
> > them again.
>
> Might as well include a fixes tag too.
>
> Fixes: abafbc551fdd ("vfio-pci: Invalidate mmaps and block MMIO access on disabled memory")
>
> You might also extend the sha1 in the log to 12 chars as well, or
> replace it with a reference to the fixes tag.
Sure.

> > Signed-off-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
> > ---
..snip..
> > diff --git a/include/linux/pci.h b/include/linux/pci.h
> > index 8355306..23a6972 100644
> > --- a/include/linux/pci.h
> > +++ b/include/linux/pci.h
> > @@ -445,6 +445,7 @@ struct pci_dev {
> >   	unsigned int	is_probed:1;		/* Device probing in progress */
> >   	unsigned int	link_active_reporting:1;/* Device capable of reporting link active */
> >   	unsigned int	no_vf_scan:1;		/* Don't scan for VFs after IOV enablement */
> > +	unsigned int	detached_vf:1;		/* VF without local PF access */
>
> Is there too much implicit knowledge in defining a "detached VF"?  For
> example, why do we know that we can skip the portion of
> vfio_config_init() that copies the vendor and device IDs from the
> struct pci_dev into the virtual config space?  It's true on s390x, but
> I think that's because we know that firmware emulates those registers
> for us.  We also skip the INTx pin register sanity checking.  Do we do
> that because we haven't installed the broken device into an s390x
> system?  Because we know firmware manages that for us too?  Or simply
> because s390x doesn't support INTx anyway, and therefore it's another
> architecture implicit decision?

That's a fair point.  This was also discussed (overnight for me) in 
another thread that this patch is very s390-specific.  It doesn't have 
to be, we could also emulate these additional pieces to make things more 
general-purpose here.

> If detached_vf is really equivalent to is_virtfn for all cases that
> don't care about referencing physfn on the pci_dev, then we should
> probably have a macro to that effect.  Otherwise, if we're just trying
> to describe that the memory bit of the command register is
> unimplemented but always enabled, like a VF, should we specifically
> describe that attribute instead?  If so, should we instead do that with
> pci_dev_flags_t?  Thanks,

Well, that's the particular issue that got us looking at this but I'm 
not so sure we wouldn't find further oddities later, hence the desire 
for a more general-purpose bit.

> Alex
>
> >   	pci_dev_flags_t dev_flags;
> >   	atomic_t	enable_cnt;	/* pci_enable_device has been called */
> >