linux-kernel - Re: [PATCH V6 0/6] Fast mmio eventfd fixes

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Date:	Tue, 15 Sep 2015 22:26:47 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Jason Wang <jasowang@...hat.com>, gleb@...nel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	cornelia.huck@...ibm.com
Subject: Re: [PATCH V6 0/6] Fast mmio eventfd fixes

On Tue, Sep 15, 2015 at 05:08:49PM +0200, Paolo Bonzini wrote:
> 
> 
> On 15/09/2015 08:41, Jason Wang wrote:
> > Hi:
> > 
> > This series fixes two issues of fast mmio eventfd:
> > 
> > 1) A single iodev instance were registerd on two buses: KVM_MMIO_BUS
> >    and KVM_FAST_MMIO_BUS. This will cause double in
> >    ioeventfd_destructor()
> > 2) A zero length iodev on KVM_MMIO_BUS will never be found but
> >    kvm_io_bus_cmp(). This will lead e.g the eventfd will be trapped by
> >    qemu instead of host.
> > 
> > 1 is fixed by allocating two instances of iodev and introduce a new
> > capability for userspace. 2 is fixed by ignore the actual length if
> > the length of iodev is zero in kvm_io_bus_cmp().
> > 
> > Please review.
> 
> Applied to kvm/queue and will send patches 1-4 for 4.3-rc.  Thanks!
> 
> Paolo

I'd prefer at least 6 to be there as well:
without 6 userspace can't safely use the code, and without 5,
it can't trace it.

> > Changes from V5:
> > - move patch of explicitly checking for KVM_MMIO_BUS to patch 1 and
> >   remove the unnecessary checks
> > - even more grammar and typo fixes
> > - rabase to kvm.git
> > - document KVM_CAP_FAST_MMIO
> > 
> > Changes from V4:
> > - move the location of kvm_assign_ioeventfd() in patch 1 which reduce
> >   the change set.
> > - commit log typo fixes
> > - switch to use kvm_deassign_ioeventfd_id) when fail to register to
> >   fast mmio bus
> > - change kvm_io_bus_cmp() as Paolo's suggestions
> > - introduce a new capability to avoid new userspace crash old kernel
> > - add a new patch that only try to register mmio eventfd on fast mmio
> >   bus
> > 
> > Changes from V3:
> > 
> > - Don't do search on two buses when trying to do write on
> >   KVM_MMIO_BUS. This fixes a small regression found by vmexit.flat.
> > - Since we don't do search on two buses, change kvm_io_bus_cmp() to
> >   let it can find zero length iodevs.
> > - Fix the unnecessary lines in tracepoint patch.
> > 
> > Changes from V2:
> > - Tweak styles and comment suggested by Cornelia.
> > 
> > Changes from v1:
> > - change ioeventfd_bus_from_flags() to return KVM_FAST_MMIO_BUS when
> >   needed to save lots of unnecessary changes.
> > 
> > Jason Wang (6):
> >   kvm: don't try to register to KVM_FAST_MMIO_BUS for non mmio eventfd
> >   kvm: factor out core eventfd assign/deassign logic
> >   kvm: fix double free for fast mmio eventfd
> >   kvm: fix zero length mmio searching
> >   kvm: add tracepoint for fast mmio
> >   kvm: add fast mmio capabilitiy
> > 
> >  Documentation/virtual/kvm/api.txt |   7 ++-
> >  arch/x86/kvm/trace.h              |  18 ++++++
> >  arch/x86/kvm/vmx.c                |   1 +
> >  arch/x86/kvm/x86.c                |   1 +
> >  include/uapi/linux/kvm.h          |   1 +
> >  virt/kvm/eventfd.c                | 124 ++++++++++++++++++++++----------------
> >  virt/kvm/kvm_main.c               |  20 +++++-
> >  7 files changed, 118 insertions(+), 54 deletions(-)
> > 
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