netdev - Re: [PATCH] vhost: add support for mandatory barriers

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Message-ID: <20210824072306.GA29073@axis.com>
Date:   Tue, 24 Aug 2021 09:23:06 +0200
From:   Vincent Whitchurch <vincent.whitchurch@...s.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
CC:     Jason Wang <jasowang@...hat.com>, kernel <kernel@...s.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] vhost: add support for mandatory barriers

On Mon, Aug 23, 2021 at 11:19:56PM +0200, Michael S. Tsirkin wrote:
> On Mon, Aug 23, 2021 at 10:14:37AM +0200, Vincent Whitchurch wrote:
> > vhost always uses SMP-conditional barriers, but these may not be
> > sufficient when vhost is used to communicate between heterogeneous
> > processors in an AMP configuration, especially since they're NOPs on
> > !SMP builds.
> > 
> > To solve this, use the virtio_*() barrier functions and ask them for
> > non-weak barriers if requested by userspace.
> > 
> > Signed-off-by: Vincent Whitchurch <vincent.whitchurch@...s.com>
> 
> I am inclined to say let's (ab)use VIRTIO_F_ORDER_PLATFORM for this.
> Jason what do you think?

OK, thanks, I'll look into that.

> Also is the use of DMA variants really the intended thing here? Could
> you point me at some examples please?

I'm using this on an ARM-based SoC.  The main processor is a Cortex-A53
(arm64) and this processor runs the virtio drivers.  The SoC also has
another processor which is a Cortex-A5 (arm32) and this processor runs
the virtio device end using vhost.  There is no coherency between these
two processors and to each other they look like any other DMA-capable
hardware.