linux-kernel - Re: [RFC] virtio: support VIRTIO_F_IO

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Message-ID: <20180503132647.yfulyzbygdfgu2or@debian>
Date:   Thu, 3 May 2018 21:26:47 +0800
From:   Tiwei Bie <tiwei.bie@...el.com>
To:     Stefan Hajnoczi <stefanha@...hat.com>
Cc:     mst@...hat.com, jasowang@...hat.com, pbonzini@...hat.com,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, dan.daly@...el.com,
        cunming.liang@...el.com, zhihong.wang@...el.com
Subject: Re: [RFC] virtio: support VIRTIO_F_IO_BARRIER

On Thu, May 03, 2018 at 10:06:52AM +0100, Stefan Hajnoczi wrote:
> On Thu, May 03, 2018 at 10:59:55AM +0800, Tiwei Bie wrote:
> > This patch introduces the support for VIRTIO_F_IO_BARRIER.
> > When this feature is negotiated, driver will use the barriers
> > suitable for hardware devices.
> > 
> > Signed-off-by: Tiwei Bie <tiwei.bie@...el.com>
> 
> I should have thought of this earlier, but why is a new feature bit
> necessary?  If a hardware virtio device is in use, then the device
> should already negotiate VIRTIO_F_IOMMU_PLATFORM (i.e. use DMA APIs and
> IOMMU callbacks).
> 
> Does disabling weak_barriers when VIRTIO_F_IOMMU_PLATFORM is set solve
> the problem?

The VIRTIO_F_IOMMU_PLATFORM feature can be set when the
device is implemented in software. And I think we don't
want the performance drop in this case.

Best regards,
Tiwei Bie