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 05:02:26PM +0300, Michael S. Tsirkin wrote:
> On Wed, Jun 20, 2018 at 09:40:50PM +0800, Tiwei Bie wrote:
> > 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..
> 
> It's a good point. Someone suggested "_I_AM_A_REAL_PCI_DEVICE".  Joking
> aside, maybe say that it's not running on another CPU
> so must not apply optimizations that assume another CPU?

Good idea! I'll try to compose a RFC ASAP.

> 
> IOMMU is somewhat special as it's used for protection
> within guests.
> 
> 
> > > 
> > > 
> > > 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
> > 
> 
> See discussion around
> 	virtio: Add platform specific DMA API translation for virito devices
> 
> you can try poking Christoph Hellwing and Benjamin Herren about
> details.

Got it. Thanks!

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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