linux-kernel - Re: [PATCH v3 3/3] vfio-pci: Invalidate mmaps and block MMIO access on disabled memory

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Date:   Tue, 26 May 2020 09:57:43 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Peter Xu <peterx@...hat.com>, John Hubbard <jhubbard@...dia.com>,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        cohuck@...hat.com, cai@....pw,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH v3 3/3] vfio-pci: Invalidate mmaps and block MMIO access
 on disabled memory

On Tue, 26 May 2020 12:53:31 -0300
Jason Gunthorpe <jgg@...pe.ca> wrote:

> On Tue, May 26, 2020 at 08:32:18AM -0600, Alex Williamson wrote:
> > > > Certainly there is no reason to optimize the fringe case of vfio
> > > > sleeping if there is and incorrect concurrnent attempt to disable the
> > > > a BAR.    
> > > 
> > > If fixup_user_fault() (which is always with ALLOW_RETRY && !RETRY_NOWAIT) is
> > > the only path for the new fault(), then current way seems ok.  Not sure whether
> > > this would worth a WARN_ON_ONCE(RETRY_NOWAIT) in the fault() to be clear of
> > > that fact.  
> > 
> > Thanks for the discussion over the weekend folks.  Peter, I take it
> > you'd be satisfied if this patch were updated as:
> > 
> > diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
> > index aabba6439a5b..35bd7cd4e268 100644
> > +++ b/drivers/vfio/pci/vfio_pci.c
> > @@ -1528,6 +1528,13 @@ static vm_fault_t vfio_pci_mmap_fault(struct vm_fault *vmf)
> >  	struct vfio_pci_device *vdev = vma->vm_private_data;
> >  	vm_fault_t ret = VM_FAULT_NOPAGE;
> >  
> > +	/*
> > +	 * We don't expect to be called with NOWAIT and there are conflicting
> > +	 * opinions on whether NOWAIT suggests we shouldn't wait for locks or
> > +	 * just shouldn't wait for I/O.
> > +	 */
> > +	WARN_ON_ONCE(vmf->flags & FAULT_FLAG_RETRY_NOWAIT);  
> 
> I don't think this is right, this implies there is some reason this
> code fails with FAULT_FLAG_RETRY_NOWAIT - but it is fine as written,
> AFAICT

Ok, Peter said he's fine either way, I'll use the patch as originally
posted and include Peter's R-b.  Thanks,

Alex