[virtio] Re: [PATCH v8 08/16] packed virtqueues: more efficient virtqueue layout

["Michael S. Tsirkin" <mst@redhat.com>]

On Sat, Feb 24, 2018 at 01:17:08PM +0800, Tiwei Bie wrote:
> On Fri, Feb 16, 2018 at 09:24:12AM +0200, Michael S. Tsirkin wrote:
> [...]
> > +\subsection{Scatter-Gather Support}
> > +\label{sec:Packed Virtqueues / Scatter-Gather Support}
> > +
> > +Some drivers need an ability to supply a list of multiple buffer
> > +elements (also known as a scatter/gather list) with a request.
> > +Two optional features support this: descriptor
> > +chaining and indirect descriptors.
> > +
> > +If neither feature has been negotiated, each buffer is
> > +physically-contigious, either read-only or write-only and is
> > +described completely by a single descriptor.
> 
> Based on the updates in "Descriptor Chaining" section
> and the below note in cover letter:
> 
> - dropped _F_DESC_LIST, 1.0 includes this unconditionally, we
>   can do same
> 
> Descriptor chaining is always available.

That's right - I should fix this.

> > +
> [...]
> > +\subsection{Event Suppression Structure Format}\label{sec:Basic
> > +Facilities of a Virtio Device / Packed Virtqueues / Event Suppression Structure
> > +Format}
> > +
> > +The following structure is used to reduce the number of
> > +notifications sent between driver and device.
> > +
> > +\begin{lstlisting}
> > +__le16 desc_event_off : 15; /* Descriptor Event Offset */
> > +int    desc_event_wrap : 1; /* Descriptor Event Wrap Counter */
> 
> Is this `int` a typo?

It's a single bit so I think it does not matter.
What type would you like me to use instead?

> > +__le16 desc_event_flags : 2; /* Descriptor Event Flags */
> > +\end{lstlisting}
> [...]
> > +\subsubsection{Implementation Example}\label{sec:Basic Facilities of a Virtio Device / Packed Virtqueues / Supplying Buffers to The Device / Implementation Example}
> > +
> > +Below is an example driver code. It does not attempt to reduce
> > +the number of device interrupts, neither does it support
> > +the VIRTIO_F_RING_EVENT_IDX feature.
> > +
> > +\begin{lstlisting}
> > +
> > +first = vq->next_avail;
> > +id = alloc_id(vq);
> > +
> > +for (each buffer element b) {
> > +        vq->desc[vq->next_avail].address = get_addr(b);
> > +        vq->desc[vq->next_avail].len = get_len(b);
> > +        init_desc(vq->next_avail, b);
> > +        avail = vq->avail_wrap_count;
> > +        used = !vq->avail_wrap_count;
> > +        f = get_flags(b) | (avail << VIRTQ_DESC_F_AVAIL) | (used << VIRTQ_DESC_F_USED);
> 
> In this version, above two flags are defined as:
> 
> #define VIRTQ_DESC_F_AVAIL     (1 << 7)
> #define VIRTQ_DESC_F_USED      (1 << 15)
> 
> So we couldn't use them to shift the bit like this.

You are right. I guess we should change all flags to be bit numbers.

> > +	/* Don't mark the 1st descriptor available until all of them are ready. */
> > +        if (vq->next_avail == first) {
> > +                flags = f;
> > +        } else {
> > +                vq->desc[vq->next_avail].flags = f;
> > +        }
> > +
> > +	vq->next_avail++;
> > +
> > +	if (vq->next_avail >= vq->size) {
> > +		vq->next_avail = 0;
> > +		vq->avail_wrap_count \^= 1;
> > +	}
> 
> Maybe it's better to not mix using tab and space in
> this example code.

I'm not sure it matters for latex but I agree we should be
consistent.

> > +
> > +
> > +}
> > +/* ID included in the last descriptor in the list */
> > +vq->desc[vq->next_avail].id = id;
> 
> Maybe it should be something like this:
> 
> vq->desc[prev].id = id;
> 
> Otherwise, the other fields (e.g. flags) of this desc (the
> vq->desc[vq->next_avail]) also need to be updated.
> 
> > +write_memory_barrier();
> > +vq->desc[first].flags = flags;
> > +
> > +memory_barrier();
> > +
> > +if (vq->device_event.flags != 0x2) {
> 
> Maybe it should be:
> 
> if (vq->device_event.flags != 0x1) {
> 
> As the flags definitions in this version are:
> 
> 00b enable events
> 01b disable events
> 10b enable events for a specific descriptor
>     (as specified by Descriptor Ring Change Event Offset/Wrap Counter).
>     Only valid if VIRTIO_F_RING_EVENT_IDX has been negotiated.
> 11b reserved

I'll recheck it and reply to above two separately.


> > +        notify_device(vq);
> > +}
> > +
> > +\end{lstlisting}
> > +
> > +
> > +\drivernormative{\paragraph}{Notifying The Device}{Basic Facilities of a Virtio Device / Packed Virtqueues / Supplying Buffers to The Device / Notifying The Device}
> > +The driver MUST perform a suitable memory barrier before reading
> > +the Driver Event Suppression structure, to avoid missing a notification.
> > +
> > +\subsection{Receiving Used Buffers From The Device}\label{sec:Basic Facilities of a Virtio Device / Packed Virtqueues / Receiving Used Buffers From The Device}
> > +
> > +Once the device has used buffers referred to by a descriptor (read from or written to them, or
> > +parts of both, depending on the nature of the virtqueue and the
> > +device), it interrupts the driver
> > +as detailed in section \ref{sec:Basic
> > +Facilities of a Virtio Device / Packed Virtqueues / Event
> > +Suppression Structure Format}.
> > +
> > +\begin{note}
> > +For optimal performance, a driver MAY disable interrupts while processing
> > +the used buffers, but beware the problem of missing interrupts between
> > +emptying the ring and reenabling interrupts.  This is usually handled by
> > +re-checking for more used buffers after interrups are re-enabled:
> > +\end{note}
> > +
> > +\begin{lstlisting}
> > +vq->driver_event.flags = 0x2;
> 
> If my understanding is correct, this is to disable interrupt.
> So maybe it should be:
> 
> vq->driver_event.flags = 0x1;

I'll check.

> > +
> > +for (;;) {
> > +        struct virtq_desc *d = vq->desc[vq->next_used];
> > +
> > +        flags = d->flags;
> > +        bool avail = flags & (1 << VIRTQ_DESC_F_AVAIL);
> > +        bool used = flags & (1 << VIRTQ_DESC_F_USED);
> 
> We couldn't use them to shift the bit like this in this version.
> 
> > +
> > +        if (avail != used) {
> > +                vq->driver_event.flags = 0x1;
> 
> If my understanding is correct, this is to enable interrupt.
> So maybe it should be:
> 
> vq->driver_event.flags = 0x0;
> 
> > +                memory_barrier();
> > +
> > +                flags = d->flags;
> > +                bool avail = flags & (1 << VIRTQ_DESC_F_AVAIL);
> > +                bool used = flags & (1 << VIRTQ_DESC_F_USED);
> 
> We couldn't use them to shift the bit like this in this version.
> 
> > +                if (avail != used) {
> > +                        break;
> > +                }
> > +
> > +                vq->driver_event.flags = 0x2;
> 
> If my understanding is correct, this is to disable interrupt.
> So maybe it should be:
> 
> vq->driver_event.flags = 0x1;


Thanks for all the comments, will address.

> > +        }
> > +
> > +	read_memory_barrier();
> > +        process_buffer(d);
> > +        vq->next_used++;
> > +        if (vq->next_used >= vq->size) {
> > +                vq->next_used = 0;
> > +        }
> > +}
> > +\end{lstlisting}
> > -- 
> > MST
> > 

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