Re: [Qemu-devel] [PATCH v7 RFC] block/vxhs: Initial commit to add Veritas HyperScale VxHS block device support

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Thu, Nov 24, 2016 at 05:44:37AM +0000, Ketan Nilangekar wrote:
> On 11/24/16, 4:07 AM, "Paolo Bonzini" <pbonzini@redhat.com> wrote:
> >On 23/11/2016 23:09, ashish mittal wrote:
> >> On the topic of protocol security -
> >> 
> >> Would it be enough for the first patch to implement only
> >> authentication and not encryption?
> >
> >Yes, of course.  However, as we introduce more and more QEMU-specific
> >characteristics to a protocol that is already QEMU-specific (it doesn't
> >do failover, etc.), I am still not sure of the actual benefit of using
> >libqnio versus having an NBD server or FUSE driver.
> >
> >You have already mentioned performance, but the design has changed so
> >much that I think one of the two things has to change: either failover
> >moves back to QEMU and there is no (closed source) translator running on
> >the node, or the translator needs to speak a well-known and
> >already-supported protocol.
> 
> IMO design has not changed. Implementation has changed significantly. I would propose that we keep resiliency/failover code out of QEMU driver and implement it entirely in libqnio as planned in a subsequent revision. The VxHS server does not need to understand/handle failover at all. 
> 
> Today libqnio gives us significantly better performance than any NBD/FUSE implementation. We know because we have prototyped with both. Significant improvements to libqnio are also in the pipeline which will use cross memory attach calls to further boost performance. Ofcourse a big reason for the performance is also the HyperScale storage backend but we believe this method of IO tapping/redirecting can be leveraged by other solutions as well.

By "cross memory attach" do you mean
process_vm_readv(2)/process_vm_writev(2)?

That puts us back to square one in terms of security.  You have
(untrusted) QEMU + (untrusted) libqnio directly accessing the memory of
another process on the same machine.  That process is therefore also
untrusted and may only process data for one guest so that guests stay
isolated from each other.

There's an easier way to get even better performance: get rid of libqnio
and the external process.  Move the code from the external process into
QEMU to eliminate the process_vm_readv(2)/process_vm_writev(2) and
context switching.

Can you remind me why there needs to be an external process?

Stefan

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