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Date:   Tue, 19 May 2020 21:24:30 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     Peter Xu <peterx@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, cohuck@...hat.com
Subject: Re: [PATCH 0/2] vfio/type1/pci: IOMMU PFNMAP invalidation

On Thu, May 14, 2020 at 04:55:17PM -0600, Alex Williamson wrote:
> On Thu, 14 May 2020 19:24:15 -0300
> Jason Gunthorpe <jgg@...pe.ca> wrote:
> 
> > On Thu, May 14, 2020 at 04:17:12PM -0600, Alex Williamson wrote:
> > 
> > > that much.  I think this would also address Jason's primary concern.
> > > It's better to get an IOMMU fault from the user trying to access those
> > > mappings than it is to leave them in place.  
> > 
> > Yes, there are few options here - if the pages are available for use
> > by the IOMMU and *asynchronously* someone else revokes them, then the
> > only way to protect the kernel is to block them from the IOMMUU.
> > 
> > For this to be sane the revokation must be under complete control of
> > the VFIO user. ie if a user decides to disable MMIO traffic then of
> > course the IOMMU should block P2P transfer to the MMIO bar. It is user
> > error to have not disabled those transfers in the first place.
> > 
> > When this is all done inside a guest the whole logic applies. On bare
> > metal you might get some AER or crash or MCE. In virtualization you'll
> > get an IOMMU fault.
> > 
> > > due to the memory enable bit.  If we could remap the range to a kernel
> > > page we could maybe avoid the IOMMU fault and maybe even have a crude
> > > test for whether any data was written to the page while that mapping
> > > was in place (ie. simulating more restricted error handling, though
> > > more asynchronous than done at the platform level).    
> > 
> > I'm not if this makes sense, can't we arrange to directly trap the
> > IOMMU failure and route it into qemu if that is what is desired?
> 
> Can't guarantee it, some systems wire that directly into their
> management processor so that they can "protect their users" regardless
> of whether they want or need it.  Yay firmware first error handling,
> *sigh*.  Thanks,

I feel like those system should just loose the ability to reliably
mirror IOMMU errors to their guests - trying to emulate it by scanning
memory/etc sounds too horrible.

Jason

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