Subject: Re: [PATCH V2 mlx5-next 12/14] vfio/mlx5: Implement vfio_pci driver for mlx5 devices

From: Jason Gunthorpe <[email-masked]>
Date: Tue, 26 Oct 2021 12:18:51 -0300

On Tue, Oct 26, 2021 at 08:42:12AM -0600, Alex Williamson wrote:

> > This is also why I don't like it being so transparent as it is
> > something userspace needs to care about - especially if the HW cannot
> > support such a thing, if we intend to allow that.
> 
> Userspace does need to care, but userspace's concern over this should
> not be able to compromise the platform and therefore making VF
> assignment more susceptible to fatal error conditions to comply with a
> migration uAPI is troublesome for me.

It is an interesting scenario.

I think it points that we are not implementing this fully properly.

The !RUNNING state should be like your reset efforts.

All access to the MMIO memories from userspace should be revoked
during !RUNNING

All VMAs zap'd.

All IOMMU peer mappings invalidated.

The kernel should directly block userspace from causing a MMIO TLP
before the device driver goes to !RUNNING.

Then the question of what the device does at this edge is not
relevant as hostile userspace cannot trigger it.

The logical way to implement this is to key off running and
block/unblock MMIO access when !RUNNING.

To me this strongly suggests that the extra bit is the correct way
forward as the driver is much simpler to implement and understand if
RUNNING directly controls the availability of MMIO instead of having
an irregular case where !RUNNING still allows MMIO but only until a
pending_bytes read.

Given the complexity of this can we move ahead with the current
mlx5_vfio and Yishai&co can come with some followup proposal to split
the freeze/queice and block MMIO?

Jason