Re: [RFC] VIRTIO_F_IO_BARRIER: use I/O barriers in driver

[Stefan Hajnoczi <stefanha@redhat.com>]

On Wed, Apr 25, 2018 at 04:23:12PM +0800, Tiwei Bie wrote:
> There will be hardware virtio devices in the future, which
> require drivers to use the barriers suitable for I/O device,
> compared with software virtio devices which just require
> drivers to use the barriers suitable for CPU core.
> 
> To fix the ordering issue for hardware virtio devices, add
> a new feature: VIRTIO_F_IO_BARRIER. When negotiated, driver
> will use the barriers suitable for I/O device.
> 
> Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
> ---
>  content.tex | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/content.tex b/content.tex
> index ae5fa4c..f24e320 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -5445,6 +5445,8 @@ Descriptors} and \ref{sec:Packed Virtqueues / Indirect Flag: Scatter-Gather Supp
>    that drivers pass extra data (besides identifying the Virtqueue)
>    in their device notifications.
>    See \ref{sec:Virtqueues / Driver notifications}~\nameref{sec:Virtqueues / Driver notifications}.
> +  \item[VIRTIO_F_IO_BARRIER(37)] This feature indicates that the device
> +  needs the driver to use the barriers suitable for hardware device.

s/hardware device/hardware devices/ (plural)

>  \end{description}
>  
>  \drivernormative{\section}{Reserved Feature Bits}{Reserved Feature Bits}
> @@ -5460,6 +5462,10 @@ addresses to the device.
>  
>  A driver SHOULD accept VIRTIO_F_RING_PACKED if it is offered.
>  
> +A driver SHOULD accept VIRTIO_F_IO_BARRIER if it is offered.
> +If VIRTIO_F_IO_BARRIER has been negotiated, a driver MUST use
> +the barriers suitable for hardware device.

s/hardware device/hardware devices/ (plural)

Please add a little context so the purpose is clear to readers (I
wouldn't be 100% sure what the spec means without more detail):

  Some transports require barriers to ensure hardware devices have a
  consistent view of memory.  When devices are implemented in software a
  weaker form of barrier may be sufficient and yield better performance.
  This feature bit indicates whether the full hardware barrier is
  necessary.

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