Re: [PATCH mlx5-next 2/7] vfio: Add an API to check migration state transition validity

From: Alex Williamson <alex.williamson@redhat.com>
To: Max Gurtovoy <mgurtovoy@nvidia.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>, Leon Romanovsky <leon@kernel.org>,
	Doug Ledford <dledford@redhat.com>,
	Yishai Hadas <yishaih@nvidia.com>,
	Bjorn Helgaas <bhelgaas@google.com>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	Kirti Wankhede <kwankhede@nvidia.com>, <kvm@vger.kernel.org>,
	<linux-kernel@vger.kernel.org>, <linux-pci@vger.kernel.org>,
	<linux-rdma@vger.kernel.org>, <netdev@vger.kernel.org>,
	Saeed Mahameed <saeedm@nvidia.com>,
	Cornelia Huck <cohuck@redhat.com>
Subject: Re: [PATCH mlx5-next 2/7] vfio: Add an API to check migration state transition validity
Date: Wed, 29 Sep 2021 16:44:09 -0600	[thread overview]
Message-ID: <20210929164409.3c33e311.alex.williamson@redhat.com> (raw)
In-Reply-To: <29835bf4-d094-ae6d-1a32-08e65847b52c@nvidia.com>

On Thu, 30 Sep 2021 00:48:55 +0300
Max Gurtovoy <mgurtovoy@nvidia.com> wrote:

> On 9/29/2021 7:14 PM, Jason Gunthorpe wrote:
> > On Wed, Sep 29, 2021 at 06:28:44PM +0300, Max Gurtovoy wrote:
> >  
> >>> So you have a device that's actively modifying its internal state,
> >>> performing I/O, including DMA (thereby dirtying VM memory), all while
> >>> in the _STOP state?  And you don't see this as a problem?  
> >> I don't see how is it different from vfio-pci situation.  
> > vfio-pci provides no way to observe the migration state. It isn't
> > "000b"  
> 
> Alex said that there is a problem of compatibility.
> 
> I migration SW is not involved, nobody will read this migration state.

The _STOP state has a specific meaning regardless of whether userspace
reads the device state value.  I think what you're suggesting is that
the device reports itself as _STOP'd but it's actually _RUNNING.  Is
that the compatibility workaround, create a self inconsistency?

We cannot impose on userspace to move a device from _STOP to _RUNNING
simply because the device supports the migration region, nor should we
report a device state that is inconsistent with the actual device state.

> >> Maybe we need to rename STOP state. We can call it READY or LIVE or
> >> NON_MIGRATION_STATE.  
> > It was a poor choice to use 000b as stop, but it doesn't really
> > matter. The mlx5 driver should just pre-init this readable to running.  
> 
> I guess we can do it for this reason. There is no functional problem nor 
> compatibility issue here as was mentioned.
> 
> But still we need the kernel to track transitions. We don't want to 
> allow moving from RESUMING to SAVING state for example. How this 
> transition can be allowed ?
> 
> In this case we need to fail the request from the migration SW...

_RESUMING to _SAVING seems like a good way to test round trip migration
without running the device to modify the state.  Potentially it's a
means to update a saved device migration data stream to a newer format
using an intermediate driver version.

If a driver is written such that it simply sees clearing the _RESUME
bit as an indicator to de-serialize the data stream to the device, and
setting the _SAVING flag as an indicator to re-serialize that data
stream from the device, then this is just a means to make use of
existing data paths.

The uAPI specifies a means for drivers to reject a state change, but
that risks failing to support a transition which might find mainstream
use cases.  I don't think common code should be responsible for
filtering out viable transitions.  Thanks,

Alex