linux-kernel - [RFC PATCH] Use vAPIC when doing IPI for PVHVM guests.

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Date:	Wed,  7 Oct 2015 16:21:48 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	boris.ostrovsky@...cle.com, joao.m.martins@...cle.com,
	david.vrabel@...rix.com, jgross@...e.com,
	dario.faggioli@...rix.com, xen-devel@...ts.xenproject.org,
	linux-kernel@...r.kernel.org
Subject: [RFC PATCH] Use vAPIC when doing IPI for PVHVM guests.

Hey,

I was running some tools in which we would heavily do rescheduling
of events - and realized to my surprise that the event channels (and
the hypercall) would slow things down. If I used the vAPIC with its
IPI support (so no VMEXIT) I got much much better performance.

Now this is an RFC because:
 1). I hadn't verified from the xentrace  how much less VMEXITS we get.
    But I remember Boris's patches and they gave at least 10%.
    I think this will get the same performance or even better.

 2). I don't know what to do with migration. That is if the guest
    migrates to older hardware it needs to recheck this I presume?

 3). Should this be enabled by default? I did get better performance
   but that was synthetic.

Thoughts?


 Documentation/kernel-parameters.txt | 15 +++++------
 arch/x86/xen/enlighten.c            | 50 +++++++++++++++++++++++++++++++++----
 arch/x86/xen/smp.c                  | 22 ++++++++++++++--
 arch/x86/xen/spinlock.c             | 18 ++++---------
 arch/x86/xen/xen-ops.h              |  4 +++
 5 files changed, 82 insertions(+), 27 deletions(-)

Konrad Rzeszutek Wilk (2):
      xen/apic: Use vAPIC for IPI if the hardware supports it.
      xen_nopv: Combine a bunch of the PV features that can be disabled

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