linux-kernel - Re: [PATCH RFC] kvm: add PV MMIO EVENTFD

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Date:	Sun, 7 Apr 2013 11:43:01 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	Alexander Graf <agraf@...e.de>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	Takuya Yoshikawa <yoshikawa_takuya_b1@....ntt.co.jp>,
	Alex Williamson <alex.williamson@...hat.com>,
	Will Deacon <will.deacon@....com>,
	Christoffer Dall <c.dall@...tualopensystems.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH RFC] kvm: add PV MMIO EVENTFD

On Sun, Apr 07, 2013 at 12:30:38PM +0300, Gleb Natapov wrote:
> On Thu, Apr 04, 2013 at 04:14:57PM +0300, Gleb Natapov wrote:
> > > > 
> > > >>> is to move to MMIO only when PIO address space is exhausted. For PCI it
> > > >>> will be never, for PCI-e it will be after ~16 devices.
> > > >> 
> > > >> Ok, let's go back a step here. Are you actually able to measure any speed in performance with this patch applied and without when going through MMIO kicks?
> > > >> 
> > > >> 
> > > > That's the question for MST. I think he did only micro benchmarks till
> > > > now and he already posted his result here:
> > > > 
> > > > mmio-wildcard-eventfd:pci-mem 3529
> > > > mmio-pv-eventfd:pci-mem 1878
> > > > portio-wildcard-eventfd:pci-io 1846
> > > > 
> > > > So the patch speedup mmio by almost 100% and it is almost the same as PIO.
> > > 
> > > Those numbers don't align at all with what I measured.
> > I am trying to run vmexit test on AMD now, but something does not work
> > there. Next week I'll fix it and see how AMD differs, bit on Intel those are the
> > numbers.
> > 
> The numbers are:
> vmcall 1921
> inl_from_kernel 4227
> outl_to_kernel 2345
> 
> outl is specifically optimized to not go through the emulator since it
> is used for virtio kick. mmio-pv-eventfd is the same kind of
> optimization but for mmio.
>  
> --
> 			Gleb.


Hmm so on AMD it's more like 20% overhead, like ARM.

-- 
MST
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