linux-kernel - Re: [RFC] virtio: use mandatory barriers for remote processor vdevs

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Date:	Thu, 1 Dec 2011 00:43:08 +0200
From:	Ohad Ben-Cohen <ohad@...ery.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [RFC] virtio: use mandatory barriers for remote processor vdevs

On Wed, Nov 30, 2011 at 4:50 PM, Michael S. Tsirkin <mst@...hat.com> wrote:
> make headers_install
> make -C tools/virtio/
> (you'll need an empty stub for tools/virtio/linux/module.h,
>  I just sent a patch to add that)
> sudo insmod tools/virtio/vhost_test/vhost_test.ko
> ./tools/virtio/virtio_test

Ok, I gave this a spin.

I've tried to see if reverting d57ed95 has any measurable effect on
the execution time of virtio_test's run_test(), but I couldn't see any
(several attempts with and without d57ed95 yielded very similar range
of execution times).

YMMV though, especially with real workloads.

> Real virtualization/x86 can keep using current smp_XX barriers, right?

Yes, sure. ARM virtualization can too, since smp_XX barriers are
enough for that scenario.

> We can have some config for your kind of setup.

Please note that it can't be a compile-time decision though (unless
we're willing to effectively revert d57ed95 when this config kicks
in): it's not unlikely that one would want to have both use cases
running on the same time.

Thanks,
Ohad.
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