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

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Date:   Thu, 3 May 2018 14:50:07 +0100
From:   Stefan Hajnoczi <stefanha@...il.com>
To:     Tiwei Bie <tiwei.bie@...el.com>
Cc:     Stefan Hajnoczi <stefanha@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>,
        "Wang, Zhihong" <zhihong.wang@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [RFC] virtio: support VIRTIO_F_IO_BARRIER

On Thu, May 3, 2018 at 2:26 PM, Tiwei Bie <tiwei.bie@...el.com> wrote:
> 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.

Good point.

Stefan