linux-kernel - Re: [PATCH 1/4] kvm: Destroy & free KVM devices on release

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Date:	Wed, 30 Oct 2013 09:56:34 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Gleb Natapov <gleb@...hat.com>, kvm@...r.kernel.org, aik@...abs.ru,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] kvm: Destroy & free KVM devices on release

On Wed, 2013-10-30 at 16:42 +0100, Paolo Bonzini wrote:
> Il 30/10/2013 16:33, Gleb Natapov ha scritto:
> > > Hmm, ok.  In that case I can drop this patch and I think the rest just
> > > boils down to userspace use of the device.  I had been close()'ing the
> > > kvm device fd when all QEMU vfio devices are detached, but I can just as
> > > easily leave it open in case a new device is added later.  I'll send out
> > > a new series after doing some more review and testing.  Do you have any
> > > comments on the rest of the series?  Thanks,
> >
> > If I understand 4/4 correctly if there is VFIO device connected we
> > assume non coherent domain. How hard it would be to do proper checking
> > in this path series?
> 
> Yes, that's my understanding as well.  Is the performance impact measurable?

I have additional patches to do this, the blocker is that intel-iommu
strips IOMMU_CACHE from the flags I provide if the IOMMU domain as a
whole (ie. all of the hardware units involved in the domain) do not all
support Snoop Control (SC).  Thus I can't rely on simply tracking the
hardware capabilities of the domain because some IOMMU PTEs will have
SNP set, others will not.  It depends on the state of the IOMMU domain
at the time of the mapping.  Therefore the only way to switch back to
coherent from non-coherent is to unmap and remap everything.  This is
what legacy KVM device assignment does, but I can't see how that's not
racy wrt inflight DMA.

The patch approach I was taking is:

1) Enable KVM to handle the VM as non-coherent (which is trued, VFIO
never sets IOMMU_CACHE currently due to the above issues).

2) Get QEMU to enable the KVM device, fixing the coherency issue.

3) Fix Intel IOMMU to honor IOMMU_CACHE regardless of hw capabilities
(patch posted).

4) Make VFIO always map w/ IOMMU_CACHE

5) Create VFIO external user interface to query capabilities.

6) Update KVM device to use it.

As to performance, for graphics I can't tell a difference whether
NoSnoop is prevented or KVM does WBINVD.  I'm hoping though that we can
consider the mode enabled by this patch as a temporary step in the
process and we'll eventually get to a state where we only emulate WBINVD
when necessary.  Correctness trumps performance in the current round.
Thanks,

Alex

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