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

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Date:   Mon, 25 Oct 2021 09:29:38 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     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 Thu, Oct 21, 2021 at 03:47:29PM -0600, Alex Williamson wrote:
> I recall that we previously suggested a very strict interpretation of
> clearing the _RUNNING bit, but again I'm questioning if that's a real
> requirement or simply a nice-to-have feature for some undefined
> debugging capability.  In raising the p2p DMA issue, we can see that a
> hard stop independent of other devices is not really practical but I
> also don't see that introducing a new state bit solves this problem any
> more elegantly than proposed here.  Thanks,

I still disagree with this - the level of 'frozenness' of a device is
something that belongs in the defined state exposed to userspace, not
as a hidden internal state that userspace can't see.

It makes the state transitions asymmetric between suspend/resume as
resume does have a defined uAPI state for each level of frozeness and
suspend does not.

With the extra bit resume does:
  
  0000, 0100, 1000, 0001

And suspend does:

  0001, 1001, 0010, 0000

However, without the extra bit suspend is only
  
  001,  010, 000

With hidden state inside the 010

Jason