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Date:   Tue, 27 Jul 2021 12:53:09 -0600
From:   Alex Williamson <alex.williamson@redhat.com>
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     Cornelia Huck <cohuck@redhat.com>, Christoph Hellwig <hch@lst.de>,
        Kirti Wankhede <kwankhede@nvidia.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] vfio/mdev: don't warn if ->request is not set
Message-ID: <20210727125309.292b30c0.alex.williamson@redhat.com>
In-Reply-To: <20210727173209.GG1721383@nvidia.com>
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        <20210726230906.GD1721383@nvidia.com>
        <20210726172831.3a7978fd.alex.williamson@redhat.com>
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On Tue, 27 Jul 2021 14:32:09 -0300
Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Tue, Jul 27, 2021 at 08:04:16AM +0200, Cornelia Huck wrote:
> > On Mon, Jul 26 2021, Alex Williamson <alex.williamson@redhat.com> wrote:
> >   
> > > On Mon, 26 Jul 2021 20:09:06 -0300
> > > Jason Gunthorpe <jgg@nvidia.com> wrote:
> > >  
> > >> On Mon, Jul 26, 2021 at 07:07:04PM +0200, Cornelia Huck wrote:
> > >>   
> > >> > But I wonder why nobody else implements this? Lack of surprise removal?    
> > >> 
> > >> The only implementation triggers an eventfd that seems to be the same
> > >> eventfd as the interrupt..
> > >> 
> > >> Do you know how this works in userspace? I'm surprised that the
> > >> interrupt eventfd can trigger an observation that the kernel driver
> > >> wants to be unplugged?  
> > >
> > > I think we're talking about ccw, but I see QEMU registering separate
> > > eventfds for each of the 3 IRQ indexes and the mdev driver specifically
> > > triggering the req_trigger...?  Thanks,
> > >
> > > Alex  
> > 
> > Exactly, ccw has a trigger for normal I/O interrupts, CRW (machine
> > checks), and this one.  
> 
> If it is a dedicated eventfd for 'device being removed' why is it in
> the CCW implementation and not core code?

The CCW implementation (likewise the vfio-pci implementation) owns the
IRQ index address space and the decision to make this a signal to
userspace rather than perhaps some handling a device might be able to
do internally.  For instance an alternate vfio-pci implementation might
zap all mmaps, block all r/w access, and turn this into a surprise
removal.  Another implementation might be more aggressive to sending
SIGKILL to the user process.  This was the thought behind why vfio-core
triggers the driver request callback with a counter, leaving the policy
to the driver.

> Is PCI doing the same?

Yes, that's where this handling originated.  Thanks,

Alex