Re: [virtio-dev] Re: [virtio] [PATCH RFC] VIRTIO_F_PARTIAL_ORDER for page fault handling

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Aug 11, 2020 at 02:53:39PM +0000, Lars Ganrot wrote:
> Hi Michael,
> 
> Short comment below.
> 
> BR,
> 
> -Lars
> 
> > From: virtio-dev@lists.oasis-open.org <virtio-dev@lists.oasis-open.org> On
> > Behalf Of Michael S. Tsirkin
> > Sent: 11. august 2020 10:23
> >
> > On Mon, Aug 10, 2020 at 06:59:28PM +0200, Cornelia Huck wrote:
> > > On Mon, 10 Aug 2020 12:15:15 -0400
> > > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > >
> > > > Devices that normally use buffers in order can benefit from ability
> > > > to temporarily switch to handle some buffers out of order.
> > > >
> > > > As a case in point, a networking device might handle RX buffers in
> > > > order normally. However, should an access to an RX buffer cause a
> > > > page fault (e.g. when using PRI), the device could benefit from
> > > > ability to temporarily keep using following buffers in the ring
> > > > (possibly with higher overhead) until the fault has been resolved.
> > > >
> > > > Page faults allow more features such as THP, auto-NUMA, live
> > > > migration.
> > > >
> > > > Out of order is of course already possible, however, IN_ORDER is
> > > > currently required for descriptor batching where device marks a
> > > > whole batch of buffers used in one go.
> > > >
> > > > The idea behind this proposal is to relax that requirement, allowing
> > > > batching without asking device to be in orde rat all times, as
> > > > follows:
> > > >
> > > > Device uses buffers in any order. Eventually when device detects
> > > > that it has used all previously outstanding buffers, it sets a FLUSH
> > > > flag on the last buffer used. If it set this flag on the last buffer
> > > > used previously, and now uses a batch of descriptors in-order, it
> > > > can now signal the last buffer used again setting the FLUSH flag.
> > > >
> > > > Driver can detect in-order when it sees two FLUSH flags one after
> > > > another. In other respects the feature is similar to IN_ORDER from
> > > > the driver implementation POV.
> > > >
> > > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > > > ---
> > > >  content.tex     |  9 ++++++++-
> > > >  packed-ring.tex | 23 +++++++++++++++++++++++  split-ring.tex  | 26
> > > > ++++++++++++++++++++++++--
> > > >  3 files changed, 55 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/content.tex b/content.tex index 91735e3..8494eb6 100644
> > > > --- a/content.tex
> > > > +++ b/content.tex
> > > > @@ -296,7 +296,11 @@ \section{Virtqueues}\label{sec:Basic Facilities
> > > > of a Virtio Device / Virtqueues}
> > > >
> > > >  Some devices always use descriptors in the same order in which
> > > > they have been made available. These devices can offer the
> > > > -VIRTIO_F_IN_ORDER feature. If negotiated, this knowledge
> > > > +VIRTIO_F_IN_ORDER feature.  Other devices sometimes use descriptors
> > > > +in the same order in which they have been made available. These
> > > > +devices can offer the VIRTIO_F_PARTIAL_ORDER feature. If one of the
> > > > +features VIRTIO_F_IN_ORDER or VIRTIO_F_PARTIAL_ORDER is
> > negotiated,
> > > > +this knowledge
> > >
> > > Do these two features conflict with each other? I.e., at most one of
> > > them may be negotiated (or offered?) at a time?
> >
> > Good point. I think so, yes. Will document.
> 
> Isn't it more natural to think of VIRTIO_F_IN_ORDER as the simple case
> which always maintains ordered access, while the new feature flag
> allows active control of when descriptors are ordered and when not? To
> make it backward compatible let VIRTIO_F_IN_ORDER imply the new bit is
> set, while the new bit set by itself without VIRTIO_F_IN_ORDER set
> means only active control is offered. I guess a name like
> VIRTIO_F_CTRL_ORDER would be more appropriate with this
> interpretation.


So how does the behaviour with both VIRTIO_F_IN_ORDER with VIRTIO_F_PARTIAL_ORDER
differ from just VIRTIO_F_IN_ORDER? If all buffers are used in order,
then what is the extra meaning of FLUSH?



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I think we are supposed to disregard this part as irrlevant, but just a
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-- 
MST