Re: [Qemu-devel] [PATCH 03/10] intel-iommu: add iommu lock

[Peter Xu <peterx@redhat.com>]

On Fri, Apr 27, 2018 at 01:13:02PM +0800, Jason Wang wrote:
> 
> 
> On 2018年04月25日 12:51, Peter Xu wrote:
> > Add a per-iommu big lock to protect IOMMU status.  Currently the only
> > thing to be protected is the IOTLB cache, since that can be accessed
> > even without BQL, e.g., in IO dataplane.
> > 
> > Note that device page tables should not need any protection.  The safety
> > of that should be provided by guest OS.  E.g., when a page entry is
> > freed, the guest OS should be responsible to make sure that no device
> > will be using that page any more.
> > 
> > Reported-by: Fam Zheng<famz@redhat.com>
> > Signed-off-by: Peter Xu<peterx@redhat.com>
> > ---
> >   include/hw/i386/intel_iommu.h |  8 ++++++++
> >   hw/i386/intel_iommu.c         | 31 +++++++++++++++++++++++++++++--
> >   2 files changed, 37 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/hw/i386/intel_iommu.h b/include/hw/i386/intel_iommu.h
> > index 220697253f..1a8ba8e415 100644
> > --- a/include/hw/i386/intel_iommu.h
> > +++ b/include/hw/i386/intel_iommu.h
> > @@ -262,6 +262,14 @@ struct IntelIOMMUState {
> >       uint8_t w1cmask[DMAR_REG_SIZE]; /* RW1C(Write 1 to Clear) bytes */
> >       uint8_t womask[DMAR_REG_SIZE];  /* WO (write only - read returns 0) */
> >       uint32_t version;
> > +    /*
> > +     * Protects IOMMU states in general.  Normally we don't need to
> > +     * take this lock when we are with BQL held.  However we have code
> > +     * paths that may run even without BQL.  In those cases, we need
> > +     * to take the lock when we have access to IOMMU state
> > +     * informations, e.g., the IOTLB.
> > +     */
> > +    QemuMutex iommu_lock;
> 
> Some questions:
> 
> 1) Do we need to protect context cache too?

IMHO the context cache entry should work even without lock.  That's a
bit trickly since we have two cases that this cache will be updated:

  (1) first translation of the address space of a device
  (2) invalidation of context entries

For (2) IMHO we don't need to worry about since guest OS should be
controlling that part, say, device should not be doing any translation
(DMA operations) when the context entry is invalidated.

For (1) the worst case is that the context entry cache be updated
multiple times with the same value by multiple threads.  IMHO that'll
be fine too.

But yes for sure we can protect that too with the iommu lock.

> 2) Can we just reuse qemu BQL here?

I would prefer not.  As I mentioned, at least I have spent too much
time on fighting BQL already.  I really hope we can start to use
isolated locks when capable.  BQL is always the worst choice to me.

> 3) I think the issue is common to all other kinds of IOMMU, so can we simply
> synchronize before calling ->translate() in memory.c. This seems a more
> common solution.

I suspect Power and s390 live well with that.  I think it mean at
least these platforms won't have problem in concurrency.  I'm adding
DavidG in loop in case there is further comment.  IMHO we should just
make sure IOMMU code be thread safe, and we fix problem if there is.

Thanks,

-- 
Peter Xu