[PATCH RFC] VIRTIO_F_PARTIAL_ORDER for page fault handling

On Mon, Aug 17, 2020 at 08:11:36AM +0000, Lars Ganrot wrote:
>
>
From:  <-
>
> 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: 
> > > > <- open.org> On Behalf Of Cornelia Huck
> > >
> Sent: 12. august 2020 14:51
> > > >
> > >
> On Tue, 11 Aug 2020 15:43:44 +0000
> > >
> Lars Ganrot <> wrote:
> > > >
> > > > >
>
From: 
> > > > > > <- open.org> On Behalf Of Lars Ganrot
> > > > >
> Sent: 11. august 2020 16:54
> > > > > >
> > > > > >
>
From: 
> > > > > > > <> 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"
> > > > > > > > <> 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 <>
> > > > > > > > > ---
> > > > > > > >
>  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?

Yes, I think so.
> The first write to a Used structure (after creation
> of the Virtqueue) would not have 2 FLUSH-flags, but would still fulfil
> the condition.

Needs thought.

> >
> 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.
> > >
> > > >
> > >
> This publicly archived list offers a means to provide input to the
> > >
> OASIS Virtual I/O Device (VIRTIO)

TC.
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