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 12:53:31 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Alex Williamson <alex.williamson@...hat.com>
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, 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

Jason