netdev - Re: [PATCH V6 mlx5-next 09/15] vfio: Extend the device migration protocol with RUNNING

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Date:   Wed, 2 Feb 2022 16:54:44 -0700
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Yishai Hadas <yishaih@...dia.com>, bhelgaas@...gle.com,
        saeedm@...dia.com, linux-pci@...r.kernel.org, kvm@...r.kernel.org,
        netdev@...r.kernel.org, kuba@...nel.org, leonro@...dia.com,
        kwankhede@...dia.com, mgurtovoy@...dia.com, maorg@...dia.com
Subject: Re: [PATCH V6 mlx5-next 09/15] vfio: Extend the device migration
 protocol with RUNNING_P2P

On Tue, 1 Feb 2022 15:50:03 -0400
Jason Gunthorpe <jgg@...dia.com> wrote:

> On Tue, Feb 01, 2022 at 12:13:22PM -0700, Alex Williamson wrote:
> > On Tue, 1 Feb 2022 14:53:21 -0400
> > Jason Gunthorpe <jgg@...dia.com> wrote:
> >   
> > > On Tue, Feb 01, 2022 at 11:31:44AM -0700, Alex Williamson wrote:  
> > > > > +	bool have_p2p = device->migration_flags & VFIO_MIGRATION_P2P;
> > > > > +
> > > > >  	if (cur_fsm >= ARRAY_SIZE(vfio_from_fsm_table) ||
> > > > >  	    new_fsm >= ARRAY_SIZE(vfio_from_fsm_table))
> > > > >  		return VFIO_DEVICE_STATE_ERROR;
> > > > >  
> > > > > -	return vfio_from_fsm_table[cur_fsm][new_fsm];
> > > > > +	if (!have_p2p && (new_fsm == VFIO_DEVICE_STATE_RUNNING_P2P ||
> > > > > +			  cur_fsm == VFIO_DEVICE_STATE_RUNNING_P2P))
> > > > > +		return VFIO_DEVICE_STATE_ERROR;    
> > > > 
> > > > new_fsm is provided by the user, we pass set_state.device_state
> > > > directly to .migration_set_state.  We should do bounds checking and
> > > > compatibility testing on the end state in the core so that we can    
> > > 
> > > This is the core :)  
> > 
> > But this is the wrong place, we need to do it earlier rather than when
> > we're already iterating next states.  I only mention core to avoid that
> > I'm suggesting a per driver responsibility.  
> 
> Only the first vfio_mig_get_next_state() can return ERROR, once it
> succeeds the subsequent ones must also succeed.

Yes, I see that.

> This is the earliest can be. It is done directly after taking the lock
> that allows us to read the current state to call this function to
> determine if the requested transition is acceptable.

I think the argument here is that there's no value to validating or
bounds checking the end state, which could be done in the core ioctl
before calling the driver if the first iteration will already fail for
both the end state and the full path validation.

> > > Userspace can never put the device into error. As the function comment
> > > says VFIO_DEVICE_STATE_ERROR is returned to indicate the arc is not
> > > permitted. The driver is required to reflect that back as an errno
> > > like mlx5 shows:
> > > 
> > > +		next_state = vfio_mig_get_next_state(vdev, mvdev->mig_state,
> > > +						     new_state);
> > > +		if (next_state == VFIO_DEVICE_STATE_ERROR) {
> > > +			res = ERR_PTR(-EINVAL);
> > > +			break;
> > > +		}
> > > 
> > > We never get the driver into error, userspaces gets an EINVAL and no
> > > change to the device state.  
> > 
> > Hmm, subtle.  I'd argue that if we do a bounds and support check of the
> > end state in vfio_ioctl_mig_set_state() before calling
> > .migration_set_state() then we could remove ERROR from
> > vfio_from_fsm_table[] altogether and simply begin
> > vfio_mig_get_next_state() with:  
> 
> Then we can't reject blocked arcs like STOP_COPY -> PRE_COPY.

Right, I hadn't made it through to 15/, which helps to clarify how the
cur_fsm + new_fsm validate the full arc.
 
> It is setup this way to allow the core code to assert all policy, not
> just a simple validation of the next_fsm.
> 
> > Then we only get to ERROR by the driver placing us in ERROR and things
> > feel a bit more sane to me.  
> 
> This is already true.
> 
> Perhaps it is confusing using ERROR to indicate that
> vfio_mig_get_next_state() failed. Would you be happier with a -errno
> return?

Yes, it's confusing to me that next_state() returns states that don't
become the device_state.  Stuffing the next step back into cur_fsm and
using an errno for a bounds/validity/blocked-arc test would be a better
API.  Thanks,

Alex