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Re: [PATCH v10 Kernel 1/5] vfio: KABI for migration interface for device

From: Alex Williamson
Subject: Re: [PATCH v10 Kernel 1/5] vfio: KABI for migration interface for device state
Date: Wed, 8 Jan 2020 11:31:34 -0700

On Wed, 8 Jan 2020 15:59:55 +0100
Cornelia Huck <address@hidden> wrote:

> On Tue, 7 Jan 2020 11:56:02 -0700
> Alex Williamson <address@hidden> wrote:
> > On Tue, 7 Jan 2020 23:23:17 +0530
> > Kirti Wankhede <address@hidden> wrote:  
> > > There are 3 invalid states:
> > >   *  101b => Invalid state
> > >   *  110b => Invalid state
> > >   *  111b => Invalid state
> > > 
> > > why only 110b should be used to report error from vendor driver to 
> > > report error? Aren't we adding more confusions in the interface?    
> > 
> > I think the only chance of confusion is poor documentation.  If we
> > define all of the above as invalid and then say any invalid state
> > indicates an error condition, then the burden is on the user to
> > enumerate all the invalid states.  That's not a good idea.  Instead we
> > could say 101b (_RESUMING|_RUNNING) is reserved, it's not currently
> > used but it might be useful some day.  Therefore there are no valid
> > transitions into or out of this state.  A vendor driver should fail a
> > write(2) attempting to enter this state.
> > 
> > That leaves 11Xb, where we consider _RESUMING and _SAVING as mutually
> > exclusive, so neither are likely to ever be valid states.  Logically,
> > if the device is in a failed state such that it needs to be reset to be
> > recovered, I would hope the device is not running, so !_RUNNING (110b)
> > seems appropriate.  I'm not sure we need that level of detail yet
> > though, so I was actually just assuming both 11Xb states would indicate
> > an error state and the undefined _RUNNING bit might differentiate
> > something in the future.
> > 
> > Therefore, I think we'd have:
> > 
> >  * 101b => Reserved
> >  * 11Xb => Error
> > 
> > Where the device can only self transition into the Error state on a
> > failed device_state transition and the only exit from the Error state
> > is via the reset ioctl.  The Reserved state is unreachable.  The vendor
> > driver must error on device_state writes to enter or exit the Error
> > state and must error on writes to enter Reserved states.  Is that still
> > confusing?  
> I think one thing we could do is start to tie the meaning more to the
> actual state (bit combination) and less to the individual bits. I.e.
> - bit 0 indicates 'running',
> - bit 1 indicates 'saving',
> - bit 2 indicates 'resuming',
> - bits 3-31 are reserved. [Aside: reserved-and-ignored or
>   reserved-and-must-be-zero?]

This version specified them as:

        Bits 3 - 31 are reserved for future use. User should perform
        read-modify-write operation on this field.

The intention is that the user should not make any assumptions about
the state of the reserved bits, but should preserve them when changing
known bits.  Therefore I think it's ignored but preserved.  If we
specify them as zero, then I think we lose any chance to define them

> [Note that I don't specify what happens when a bit is set or unset.]
> States are then defined as:
> 000b => stopped state (not saving or resuming)
> 001b => running state (not saving or resuming)
> 010b => stop-and-copy state
> 011b => pre-copy state
> 100b => resuming state
> [Transitions between these states defined, as before.]
> 101b => reserved [for post-copy; no transitions defined]
> 111b => reserved [state does not make sense; no transitions defined]
> 110b => error state [state does not make sense per se, but it does not
>         indicate running; transitions into this state *are* possible]
> To a 'reserved' state, we can later assign a different meaning (we
> could even re-use 111b for a different error state, if needed); while
> the error state must always stay the error state.
> We should probably use some kind of feature indication to signify
> whether a 'reserved' state actually has a meaning. Also, maybe we also
> should designate the states > 111b as 'reserved'.
> Does that make sense?

It seems you have an opinion to restrict this particular error state to
110b rather than 11Xb, reserving 111b for some future error condition.
That's fine and I think we agree that using the state with _RUNNING set
to zero is more logical as we expect the device to be non-operational
in this state.

I'm also thinking more of these as states, but at the same time we're
not doing away with the bit definitions.  I think the states are much
easier to decode and use if we think about the function of each bit,
which leads to the logical incongruity that the 11Xb states are
impossible and therefore must be error states.

I took a look at drawing a state transitions diagram, but I think we're
fully interconnected for the 6 states we're defining.  The user can
invoke transition to any of the 5 states Connie lists above from any of
those states and the 6th error state is only reached via failed
transition and only exited via device reset, returning the user to the
running state.  There are a couple transitions of questionable value,
particularly 01Xb -> 100b (_SAVING -> _RESUMING), but I can't convince
myself that it's worthwhile to force the user to pass through another
state in order to restrict those.  Are there any cases I'm missing
where the vendor driver has good reason not to support arbitrary
transitions between the above 5 states?  Thanks,


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