linux-kernel - Re: [Qemu-devel] [RFC] Next gen kvm api

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Date:	Tue, 07 Feb 2012 10:19:56 -0600
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	Avi Kivity <avi@...hat.com>
CC:	Rob Earhart <earhart@...gle.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>,
	qemu-devel <qemu-devel@...gnu.org>
Subject: Re: [Qemu-devel] [RFC] Next gen kvm api

On 02/07/2012 10:02 AM, Avi Kivity wrote:
> On 02/07/2012 05:17 PM, Anthony Liguori wrote:
>> On 02/07/2012 06:03 AM, Avi Kivity wrote:
>>> On 02/06/2012 09:11 PM, Anthony Liguori wrote:
>>>>
>>>> I'm not so sure. ioeventfds and a future mmio-over-socketpair have to put the
>>>> kthread to sleep while it waits for the other end to process it. This is
>>>> effectively equivalent to a heavy weight exit. The difference in cost is
>>>> dropping to userspace which is really neglible these days (< 100 cycles).
>>>
>>> On what machine did you measure these wonderful numbers?
>>
>> A syscall is what I mean by "dropping to userspace", not the cost of a heavy
>> weight exit.
>
> Ah. But then ioeventfd has that as well, unless the other end is in the kernel too.

Yes, that was my point exactly :-)

ioeventfd/mmio-over-socketpair to adifferent thread is not faster than a 
synchronous KVM_RUN + writing to an eventfd in userspace modulo a couple of 
cheap syscalls.

The exception is when the other end is in the kernel and there is magic 
optimizations (like there is today with ioeventfd).

Regards,

Anthony Liguori

>
>> I think a heavy weight exit is still around a few thousand cycles.
>>
>> Any nehalem class or better processor should have a syscall cost of around
>> that unless I'm wildly mistaken.
>>
>
> That's what I remember too.
>
>>>
>>> But I agree a heavyweight exit is probably faster than a double context switch
>>> on a remote core.
>>
>> I meant, if you already need to take a heavyweight exit (and you do to
>> schedule something else on the core), than the only additional cost is taking
>> a syscall return to userspace *first* before scheduling another process. That
>> overhead is pretty low.
>
> Yeah.
>

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