Re: [Xen-devel] HYBRID: PV in HVM container

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 29 Jul 2011, Konrad Rzeszutek Wilk wrote:
> On Thu, Jul 28, 2011 at 12:34:42PM +0100, Stefano Stabellini wrote:
> > On Thu, 28 Jul 2011, Mukesh Rathor wrote:
> > > Hi folks,
> > > 
> > > Well, I did some benchmarking and found interesting results. Following
> > > runs are on a westmere with 2 sockets and 10GB RAM.  Xen was booted
> > > with maxcpus=2 and entire RAM. All guests were started with 1vcpu and 2GB 
> > > RAM. dom0 started with 1 vcpu and 704MB. Baremetal was booted with 2GB 
> > > and 1 cpu.  HVM guest has EPT enabled. HT is on.
> > > 
> > > So, unless the NUMA'ness interfered with results (using some memory on 
> > > remote socket), it appears HVM does very well. To the point that it
> > > seems a hybrid is not going to be worth it. I am currently running
> > > tests on a single socket system just to be sure.
> > > 
> > 
> > The high level benchmarks I run to compare PV and PV on HVM guests show
> > a very similar scenario.
> > 
> > It is still worth having HYBRID guests (running with EPT?) in order to
> > support dom0 in an HVM container one day not too far from now.
> 
> I am just wondering how much dom0 cares about this? I mean if you use
> blkback, netback - etc - they are all in the kernel. The device drivers
> are also in the kernel.

There are always going to be some userspace processes, even with
stubdoms.
Besides if we have HVM dom0, we can enable
XENFEAT_auto_translated_physmap and EPT and have the same level of
performances of a PV on HVM guest. Moreover since we wouldn't be using
the mmu pvops anymore we could drop them completely: that would greatly
simplify the Xen maintenance in the Linux kernel as well as gain back
some love from the x86 maintainers :)

The way I see it, normal Linux guests would be PV on HVM guests, but we
still need to do something about dom0.
This work would make dom0 exactly like PV on HVM guests apart from
the boot sequence: dom0 would still boot from xen_start_kernel,
everything else would be pretty much the same.

I would ask you to run some benchmarks using
XENFEAT_auto_translated_physmap but I am afraid it bitrotted over the
years so it would need some work to get it working.

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