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

[Lars Ganrot <lga@napatech.com>]

> From: virtio-comment@lists.oasis-open.org <virtio-comment@lists.oasis-
> open.org> On Behalf Of Michael S. Tsirkin
> Sent: 14. august 2020 01:18
> 
> On Wed, Aug 12, 2020 at 03:55:18PM +0000, Lars Ganrot wrote:
> > > From: virtio-comment@lists.oasis-open.org
> > > <virtio-comment@lists.oasis- open.org> On Behalf Of Cornelia Huck
> > > Sent: 12. august 2020 14:51
> > >
> > > On Tue, 11 Aug 2020 15:43:44 +0000
> > > Lars Ganrot <lga@napatech.com> wrote:
> > >
> > > > > From: virtio-comment@lists.oasis-open.org
> > > > > <virtio-comment@lists.oasis- open.org> On Behalf Of Lars Ganrot
> > > > > Sent: 11. august 2020 16:54
> > > > >
> > > > > > 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.
> > > > >
> > > >
> > > > On second thought that might be a bit backwards - how about:
> > > >
> > > > Legacy case: VIRTIO_F_IN_ORDER==0/1 + VIRTIO_F_ORDER_RELAX==0
> This
> > > > proposal: VIRTIO_F_IN_ORDER==1 + VIRTIO_F_ORDER_RELAX==1
> Potential
> > > > future use: VIRTIO_F_???_ORDER==1 + VIRTIO_F_ORDER_RELAX==0/1
> > >
> > > What happens in the new device/old driver case?
> > > - device offers IN_ORDER and PARTIAL_ORDER
> > > - driver does not know PARTIAL_ORDER, accepts IN_ORDER
> > > - device now only can do complete ordering
> > >
> > > Maybe I don't understand the purpose of the new feature correctly,
> > > but I thought it was for those devices that don't do full in-order,
> > > but can do it for a subset of buffers? As such, the two features
> > > can't really imply each other: a device offering IN_ORDER might not
> > > know about the new feature and its mechanism, and a device offering
> > > the new feature, but not IN_ORDER probably does so because it cannot
> support full IN_ORDER.
> > >
> > > I think it makes the most sense if the device can offer both flags,
> > > but the driver must only accept at most one of them?
> > >
> >
> > Yes, you are absolutely right. Keeping them as two mutually exclusive
> options is probably best.
> >
> > Another aspect of the proposal is that the functionality is only
> > described for the packed ring layout, but the concept should be
> > equally applicable to the split ring.
> 
> Pls take a look at the changes in split-ring.tex Are they unclear?
> 

I guess not. I failed to understand that the used-ring 32-bit ID field is divided into a 16-bit ID and a 16-bit flags. Sorry for the confusion.

Does this mean that for a device that always sets the FLUSH-flag, the PARTIAL_ORDER feature will be identical to IN_ORDER for both packed and split rings? The first write to a Used structure (after creation of the Virtqueue) would not have 2 FLUSH-flags, but would still fulfil the condition.

> > The split ring does not have
> > flags for each used ring element, so the FLUSH flag functionality
> > needs to be a bit different. One way is to add an IN_ORDER-flag to the
> > virtq_used.flags, that allows the device to signal when it starts to
> > adhere to IN_ORDER buffer use. The driver acknowledges this when it is
> > ready to fulfil its part of the IN_ORDER restrictions by setting a new
> > virtq_avail.flag: IN_ORDER_ACK. At this point both device and driver
> > can assume IN_ORDER optimization until the device clears IN_ORDER.
> > After clearing IN_ORDER, the device must wait until IN_ORDER_ACK is
> > cleared before IN_ORDER can be asserted it again.
> 
> It would be tricky to figure out which buffers does the handshake apply to. My
> proposal does it a bit differently, pls take a look.
> 
> 
> >
> > This is a little less fine-grained than the packed-ring solution with the FLUSH
> flag, but for infrequent scenarios like migration it should be OK.
> >
> > >
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