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Subject: Re: [virtio-dev] Re: [virtio] [PATCH RFC] VIRTIO_F_PARTIAL_ORDER for page fault handling
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? > > Disclaimer: This email and any files transmitted with it may contain > confidential information intended for the addressee(s) only. The > information is not to be surrendered or copied to unauthorized > persons. If you have received this communication in error, please > notify the sender immediately and delete this e-mail from your system. I think we are supposed to disregard this part as irrlevant, but just a reminder that you agreed that all material posted to the virtio mailing lists is governed by the relevant IPR rules. It is your responsibility not to post confidential information there. -- MST
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