virtio — archive

[PATCH RFC] VIRTIO_F_PARTIAL_ORDER for page fault handling

From: Cornelia Huck <[email protected]>
To: [email protected]
Date: Wed, 12 Aug 2020 12:51:00 -0000

On Tue, 11 Aug 2020 15:43:44 +0000 Lars Ganrot <[email protected]> wrote: > > From: [email protected] <[email protected] > > open.org> On Behalf Of Lars Ganrot > > Sent: 11. august 2020 16:54 > > > > > From: [email protected] > > > <[email protected]> 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" <[email protected]> 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 <[email protected]> > > > > > --- > > > > > 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?