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Subject: Re: [virtio-dev] [PATCH v3] VIRTIO_F_IO_BARRIER: use I/O barriers in driver
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 > > > > > > > > > > > > --------------------------------------------------------------------- > > > > To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org > > > > For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org
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