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Re: [Xen-devel] RFC: xen config changes v4
On 02/27/2015 01:24 PM, Stefano Stabellini wrote:
On Fri, 27 Feb 2015, Juergen Gross wrote:
On 02/27/2015 11:11 AM, Stefano Stabellini wrote:
On Fri, 27 Feb 2015, Juergen Gross wrote:
On 02/27/2015 10:41 AM, Stefano Stabellini wrote:
On Fri, 27 Feb 2015, Juergen Gross wrote:
On 02/26/2015 06:42 PM, Stefano Stabellini wrote:
On Thu, 26 Feb 2015, Luis R. Rodriguez wrote:
On Thu, Feb 26, 2015 at 11:08:20AM +0000, Stefano Stabellini
wrote:
On Thu, 26 Feb 2015, David Vrabel wrote:
On 26/02/15 04:59, Juergen Gross wrote:
So we are again in the situation that pv-drivers always
imply
the
pvops
kernel (PARAVIRT selected). I started the whole Kconfig
rework
to
eliminate this dependency.
Yes. Can you produce a series that just addresses this one
issue.
In the absence of any concrete requirement for this big
Kconfig
reorg
I
I don't think it is helpful.
I clearly missed some context as I didn't realize that this was
the
intended goal. Why do we want this? Please explain as it won't
come
for free.
We have a few PV interfaces for HVM guests that need PARAVIRT in
Linux
in order to be used, for example pv_time_ops and
HVMOP_pagetable_dying.
They are critical performance improvements and from the
interface
perspective, small enough that doesn't make much sense having a
separate
KConfig option for them.
In order to reach the goal above we necessarily need to
introduce a
differentiation in terms of PV on HVM guests in Linux:
1) basic guests with PV network, disk, etc but no PV timers, no
HVMOP_pagetable_dying, no PV IPIs
2) full PV on HVM guests that have PV network, disk, timers,
HVMOP_pagetable_dying, PV IPIs and anything else that
makes
sense.
2) is much faster than 1) on Xen and 2) is only a tiny bit
slower
than
1) on native x86
Also don't we shove 2) down hvm guests right now? Even when
everything
is
built in I do not see how we opt out for HVM for 1) at run time
right
now.
If this is true then the question of motivation for this becomes
even
stronger I think.
Yes, indeed there is no way to do 1) at the moment. And for good
reasons, see above.
Hmm, after checking the code I'm not convinced:
- HVMOP_pagetable_dying is obsolete on modern hardware supporting
EPT/HAP
That might be true, but what about older hardware?
Even on modern hardware a few workloads still run faster on shadow.
But if HVMOP_pagetable_dying is the only reason to keep PARAVIRT for HVM
guests, then I agree with you that we should remove it.
- PV IPIs are not needed on single-vcpu guests
- PARAVIRT_CLOCK doesn't need PARAVIRT (in fact the SUSEs kernel
configs
for all x86_64 kernels have CONFIG_PARAVIRT_CLOCK=y)
So I think we really should enable building Xen frontends without
PARAVIRT, implying at least no XEN_PV and no XEN_PVH.
I'll have a try setting up patches.
If we are doing this as a performance improvement, I would like to see a
couple of benchmarks (kernbench, hackbench) to show that on a
single-vcpu guest and multi-vcpu guest (let's say 4 vcpus) disabling
PARAVIRT leads to better performance on Xen on EPT hardware.
This is not meant to be a performance improvement. It is meant to enable
a standard distro kernel configured without PARAVIRT to be able to run
as a HVM guest using the pv-drivers.
This is not a convincing explanation. Debian, Ubuntu and Fedora seems
to be able to cope with it just fine.
Why do you want to do that, even though it will cause a performance
regression and a maintenance pain? You haven't provided a reason yet.
Either we are talking about different things, or I really don't
understand your problem here. I don't want to disable something. I
just want to enable kernels without PARAVIRT to run under Xen better
than today. Being it 32 bit non-PAE kernels as Ian pointed out or
distro kernels like e.g. SLES and probably RHEL.
Using PV frontends is completely orthogonal to other PV enhancements
like PARAVIRT_CLOCK, HVMOP_pagetable_dying or PV IPIs. So why do you
object enabling the PV frontends for those kernels?
I am for it. I would like to avoid two user visible XEN enablement
options (XEN_FRONTEND vs. XEN_PVHVM) for x86_64 and PAE HVM guests to
avoid configurations with just XEN_FRONTEND, that can be considered a
performance regression compared to what we have now (on x86_64 and PAE).
Would you be okay with making this an expert configuration alternative
for PAE/x86_64? This would enable the possibility to use PV drivers for
native-performance-tuned kernels. I would explicitly mention the better
alternative XEN_PVHVM in the Kconfig help text.
Juergen
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