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

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Message-ID: <CAK=WgbazBq4GVo16sZsrZirgMYbwMcWtPaqn90c6d9Phr0gh3A@mail.gmail.com>
Date:	Wed, 30 Nov 2011 18:24:16 +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 6:15 PM, Michael S. Tsirkin <mst@...hat.com> wrote:
> How are the rings mapped? normal memory, right?

No, device memory.

> We allocate them with plan alloc_pages_exact in virtio_pci.c ...

I'm not using virtio_pci.c; remoteproc is allocating the rings using
the DMA API.

> Yes wmb() is required to ensure ordering for MMIO.
> But here both accesses: index and ring - are for
> memory, not MMIO.

I'm doing IO with a device over shared memory. It does require
mandatory barriers as I explained.

> Is this something you see in practice?

Yes. These bugs are very real.
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