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Date: Mon, 13 Jul 2020 10:24:35 +0100
From: Stefan Hajnoczi <>
To: Konrad Rzeszutek Wilk <>
Cc: Jens Axboe <>, Sargun Dhillon <>,
	Kees Cook <>,,,
	Kernel Hardening <>,
	Jann Horn <>, Aleksa Sarai <>,
	Christian Brauner <>,, Alexander Viro <>,
	Jeff Moyer <>,
	Stefano Garzarella <>
Subject: Re: [PATCH RFC 0/3] io_uring: add restrictions to support untrusted
 applications and guests

On Fri, Jul 10, 2020 at 06:20:17PM +0200, Stefano Garzarella wrote:
> On Fri, Jul 10, 2020 at 11:33:09AM -0400, Konrad Rzeszutek Wilk wrote:
> > .snip..
> > > Just to recap the proposal, the idea is to add some restrictions to the
> > > operations (sqe, register, fixed file) to safely allow untrusted applications
> > > or guests to use io_uring queues.
> > 
> > Hi!
> > 
> > This is neat and quite cool - but one thing that keeps nagging me is
> > what how much overhead does this cut from the existing setup when you use
> > virtio (with guests obviously)?
> I need to do more tests, but the preliminary results that I reported on
> the original proposal [1] show an overhead of ~ 4.17 uS (with iodepth=1)
> when I'm using virtio ring processed in a dedicated iothread:
>   - 73 kIOPS using virtio-blk + QEMU iothread + io_uring backend
>   - 104 kIOPS using io_uring passthrough
> >                                 That is from a high level view the
> > beaty of io_uring being passed in the guest is you don't have the
> > virtio ring -> io_uring processing, right?
> Right, and potentially we can share the io_uring queues directly to the
> guest userspace applications, cutting down the cost of Linux block
> layer in the guest.

Another factor is that the guest submits requests directly to the host
kernel sqpoll thread. When a virtqueue is used the sqpoll thread cannot
poll it directly so another host thread (QEMU) needs to poll the
virtqueue. The same applies for the completion code path.


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