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Subject: Re: [virtio] [PATCH RFC] VIRTIO_F_PARTIAL_ORDER for page fault handling
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?
> might allow optimizations or simplify driver and/or device code.
>
> Each virtqueue can consist of up to 3 parts:
> @@ -6132,6 +6136,9 @@ \chapter{Reserved Feature Bits}\label{sec:Reserved Feature Bits}
> that the driver passes extra data (besides identifying the virtqueue)
> in its device notifications.
> See \ref{sec:Virtqueues / Driver notifications}~\nameref{sec:Virtqueues / Driver notifications}.
> + \item[VIRTIO_F_PARTIAL_ORDER(39)] This feature indicates
> + that device has ability to indicate use of (some of) buffers by the device in the same
"use of a subset of buffers" ?
> + order in which they have been made available.
> \end{description}
>
> \drivernormative{\section}{Reserved Feature Bits}{Reserved Feature Bits}
> diff --git a/packed-ring.tex b/packed-ring.tex
> index ea92543..a120a19 100644
> --- a/packed-ring.tex
> +++ b/packed-ring.tex
> @@ -284,6 +284,29 @@ \subsection{In-order use of descriptors}
> only writing out a single used descriptor with the Buffer ID
> corresponding to the last descriptor in the batch.
>
> +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 negotiated, whenever device has used all buffers
s/device/the device/
> +since the previous used buffer in the same order
> +in which they have been made available, device can set the
s/device/the device/
> +VIRTQ_DESC_F_FLUSH flag in the used descriptor.
> +\begin{lstlisting}
> +#define VIRTQ_DESC_F_FLUSH 8
> +\end{lstlisting}
> +
> +This knowledge allows
> +devices to notify the use of a batch of buffers to the driver by
> +only writing out a single used descriptor with the Buffer ID
> +corresponding to the last descriptor in the batch,
> +and VIRTQ_DESC_F_FLUSH set.
> +
> +Note that device is only allowed to batch buffers in this way
s/device/the device/
> +if the previous used descriptor also has the VIRTQ_DESC_F_FLUSH
> +flag set, as a result, considering the group of buffers
s/set, as/set. As/
> +used between two buffers with VIRTQ_DESC_F_FLUSH set,
> +either all of them constitute a batch, or none at all.
> +
> The device then skips forward in the ring according to the size of
> the batch. The driver needs to look up the used Buffer ID and
> calculate the batch size to be able to advance to where the next
> diff --git a/split-ring.tex b/split-ring.tex
> index 123ac9f..cf197f8 100644
> --- a/split-ring.tex
> +++ b/split-ring.tex
> @@ -398,10 +398,11 @@ \subsection{The Virtqueue Used Ring}\label{sec:Basic Facilities of a Virtio Devi
> le16 avail_event; /* Only if VIRTIO_F_EVENT_IDX */
> };
>
> -/* le32 is used here for ids for padding reasons. */
> struct virtq_used_elem {
> /* Index of start of used descriptor chain. */
> - le32 id;
> + le16 id;
> +#define VIRTQ_USED_ELEM_F_FLUSH 0x8000
> + le16 flags;
This is a bit confusing for legacy, as this will only work for
little-endian. The flags field will only be used for non-legacy, but I
think the id field should still be treated as 32 bit for legacy.
(We have a wholesale "it's guest endian when legacy" further up.)
> /* Total length of the descriptor chain which was used (written to) */
> le32 len;
> };
> @@ -481,6 +482,27 @@ \subsection{In-order use of descriptors}
> corresponding to the head entry of the
> descriptor chain describing the last buffer in the batch.
>
> +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 negotiated, whenever device has used all buffers
s/device/the device/
> +since the previous used buffer in the same order
> +in which they have been made available, device can set the
s/device/the device/
> +VIRTQ_USED_ELEM_F_FLUSH flag in the used ring entry.
> +
> +This knowledge allows
> +devices to notify the use of a batch of buffers to the driver by
> +only writing out single used ring entry with the \field{id}
s/single/a single/
> +corresponding to the head entry of the
> +descriptor chain describing the last buffer in the batch,
> +and VIRTQ_USED_ELEM_F_FLUSH set.
> +
> +Note that device is only allowed to batch buffers in this way
s/device/the device/
> +if the previous used ring entry also has the VIRTQ_USED_ELEM_F_FLUSH
> +flag set, as a result, considering the group of buffers
s/set, as/set. As/
> +used between two buffers with VIRTQ_USED_ELEM_F_FLUSH set,
> +either all of them constitute a batch, or none at all.
> +
> The device then skips forward in the ring according to the size of
> the batch. Accordingly, it increments the used \field{idx} by the
> size of the batch.
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