netdev - Re: [PATCH V2 mlx5-next 12/14] vfio/mlx5: Implement vfio

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Date:   Tue, 2 Nov 2021 10:22:36 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Shameerali Kolothum Thodi <shameerali.kolothum.thodi@...wei.com>,
        Cornelia Huck <cohuck@...hat.com>,
        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,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>
Subject: Re: [PATCH V2 mlx5-next 12/14] vfio/mlx5: Implement vfio_pci driver
 for mlx5 devices

On Tue, 2 Nov 2021 12:54:20 -0300
Jason Gunthorpe <jgg@...dia.com> wrote:

> On Tue, Nov 02, 2021 at 08:56:51AM -0600, Alex Williamson wrote:
> 
> > > Still, this is something that needs clear definition, I would expect
> > > the SET_IRQS to happen after resuming clears but before running sets
> > > to give maximum HW flexibility and symmetry with saving.  
> > 
> > There's no requirement that the device enters a null state (!_RESUMING
> > | !_SAVING | !_RUNNING), the uAPI even species the flows as _RESUMING
> > transitioning to _RUNNING.    
> 
> If the device saves the MSI-X state inside it's migration data (as
> apparently would be convenient for other OSs) then when RESUMING
> clears and the migration data is de-serialized the device will
> overwrite the MSI-X data.
> 
> Since Linux as an OS wants to control the MSI-X it needs to load it
> after RESUMING, but before RUNNING.

This is not how it works today, QEMU enables MSI/X based on the config
space information, which is also outside of the device migration stream.

> > There's no point at which we can do SET_IRQS other than in the
> > _RESUMING state.  Generally SET_IRQS ioctls are coordinated with the
> > guest driver based on actions to the device, we can't be mucking
> > with IRQs while the device is presumed running and already
> > generating interrupt conditions.  
> 
> We need to do it in state 000
> 
> ie resume should go 
> 
>   000 -> 100 -> 000 -> 001
> 
> With SET_IRQS and any other fixing done during the 2nd 000, after the
> migration data has been loaded into the device.

Again, this is not how QEMU works today.

> > > And we should really define clearly what a device is supposed to do
> > > with the interrupt vectors during migration. Obviously there are races
> > > here.  
> > 
> > The device should not be generating interrupts while !_RUNNING, pending
> > interrupts should be held until the device is _RUNNING.  To me this
> > means the sequence must be that INTx/MSI/MSI-X are restored while in
> > the !_RUNNING state.  
> 
> Yes

Except I suppose them to be restored while _RESUMING is set.

> > > > In any case, it requires that the device cannot be absolutely static
> > > > while !_RUNNING.  Does (_RESUMING) have different rules than
> > > > (_SAVING)?    
> > > 
> > > I'd prever to avoid all device touches during both resuming and
> > > saving, and do them during !RUNNING  
> > 
> > There's no such state required by the uAPI.  
> 
> The uAPI comment does not define when to do the SET_IRQS, it seems
> this has been missed.
> 
> We really should fix it, unless you feel strongly that the
> experimental API in qemu shouldn't be changed.

I think the QEMU implementation fills in some details of how the uAPI
is expected to work.  MSI/X is expected to be restored while _RESUMING
based on the config space of the device, there is no intermediate step
between _RESUMING and _RUNNING.  Introducing such a requirement
precludes the option of a post-copy implementation of (_RESUMING |
_RUNNING).  Thanks,

Alex