Re: [virtio-dev] [PATCH v3] VIRTIO_F_IO_BARRIER: use I/O barriers in driver

[Tiwei Bie <tiwei.bie@intel.com>]

On Wed, Jun 20, 2018 at 06:31:16AM +0300, Michael S. Tsirkin wrote:
> On Thu, May 10, 2018 at 11:41:26PM +0800, Tiwei Bie wrote:
> > There will be hardware virtio devices in the future, which
> > require drivers to use the barriers suitable for I/O devices,
> > compared with software virtio devices which just require
> > drivers to use the barriers suitable for CPU cores.
> > 
> > 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 devices.
> > 
> > Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
> > Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> 
> So some people noticed that some systems also
> require a cache flush around DMA to real devices.
> 
> Shouldn't we add that as part of VIRTIO_F_IO_BARRIER?
> 
> Also rename it?
> 

Do we just need to add cache flush? One thing
worried me is that I'm not sure how to cover
all the possible cases..

> 
> 
> Others also noticed that there are systems where accesses
> include some kind of translation e.g. some high bits
> in the address are set to signal access properties.
> Should that be included in VIRTIO_F_IOMMU_PLATFORM?
> 

It's my first time to know such systems. I don't
know any details about it..

Best regards,
Tiwei Bie


> 
> 
> 
> > ---
> > v2 -> v3:
> > - Update the feature bits allocation (Stefan);
> > 
> > v1 -> v2:
> > - Rebase to the latest spec (MST);
> > - Use a smaller textwidth (according to _vimrc);
> > 
> > RFC -> v1:
> > - Use plural (Stefan);
> > - Add more details (Stefan);
> > 
> >  content.tex | 20 ++++++++++++++++++--
> >  1 file changed, 18 insertions(+), 2 deletions(-)
> > 
> > diff --git a/content.tex b/content.tex
> > index 7a92cb1..95c243f 100644
> > --- a/content.tex
> > +++ b/content.tex
> > @@ -95,10 +95,10 @@ Feature bits are allocated as follows:
> >  \begin{description}
> >  \item[0 to 23] Feature bits for the specific device type
> >  
> > -\item[24 to 33] Feature bits reserved for extensions to the queue and
> > +\item[24 to 36] Feature bits reserved for extensions to the queue and
> >    feature negotiation mechanisms
> >  
> > -\item[34 and above] Feature bits reserved for future extensions.
> > +\item[37 and above] Feature bits reserved for future extensions.
> >  \end{description}
> >  
> >  \begin{note}
> > @@ -5348,6 +5348,15 @@ Descriptors} and \ref{sec:Packed Virtqueues / Indirect Flag: Scatter-Gather Supp
> >    \item[VIRTIO_F_IN_ORDER(35)] This feature indicates
> >    that all buffers are used by the device in the same
> >    order in which they have been made available.
> > +  \item[VIRTIO_F_IO_BARRIER(36)] This feature indicates
> > +  that the device needs the driver to use the barriers
> > +  suitable for hardware devices.  Some transports require
> > +  barriers to ensure 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 indicates whether
> > +  a stronger form of barrier suitable for hardware
> > +  devices is necessary.
> >  \end{description}
> >  
> >  \drivernormative{\section}{Reserved Feature Bits}{Reserved Feature Bits}
> > @@ -5363,6 +5372,10 @@ 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 devices.
> > +
> >  \devicenormative{\section}{Reserved Feature Bits}{Reserved Feature Bits}
> >  
> >  A device MUST offer VIRTIO_F_VERSION_1.  A device MAY fail to operate further
> > @@ -5376,6 +5389,9 @@ accepted.
> >  If VIRTIO_F_IN_ORDER has been negotiated, a device MUST use
> >  buffers in the same order in which they have been available.
> >  
> > +A device MAY fail to operate further if VIRTIO_F_IO_BARRIER
> > +is not accepted.
> > +
> >  \section{Legacy Interface: Reserved Feature Bits}\label{sec:Reserved Feature Bits / Legacy Interface: Reserved Feature Bits}
> >  
> >  Transitional devices MAY offer the following:
> > -- 
> > 2.11.0
> > 
> > 
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