Re: Intention to work on GSoC project

[Eugenio Perez Martin <eperezma@redhat.com>]

On Tue, Apr 2, 2024 at 6:58 AM Sahil <icegambit91@gmail.com> wrote:
>
> Hi,
>
> On Monday, April 1, 2024 11:53:11 PM IST daleyoung4242@gmail.com wrote:
> > Hi,
> >
> > On Monday, March 25, 2024 21:20:32 CST Sahil wrote:
> > > Q1.
> > > Section 2.7.4 of the virtio spec [3] states that in an available
> > > descriptor, the "Element Length" stores the length of the buffer element.
> > > In the next few lines, it also states that the "Element Length" is
> > > reserved for used descriptors and is ignored by drivers. This sounds a
> > > little contradictory given that drivers write available desciptors in the
> > > descriptor ring.
> > When VIRTQ_DESC_F_WRITE is set, the device will use "Element Length" to
> > specify the length it writes. When VIRTQ_DESC_F_WRITE is not set, which
> > means the buffer is read-only for the device, "Element Length" will not be
> > changed by the device, so drivers just ignore it.
>
>
> Thank you for the clarification. I think I misunderstood what I had read
> in the virtio spec. What I have understood now is that "Element Length"
> has different meanings for available and used descriptors.
>
> Correct me if I am wrong - for available descriptors, it represents the
> length of the buffer. For used descriptors, it represents the length of
> the buffer that is written to by the device if it's write-only, otherwise it
> has no meaning and hence can be ignored by drivers.
>

Both answers are correct.

> > > Q2.
> > > In the Red Hat article, just below the first listing ("Memory layout of a
> > > packed virtqueue descriptor"), there's the following line referring to the
> > > buffer id in
> > >
> > > "virtq_desc":
> > > > This time, the id field is not an index for the device to look for the
> > > > buffer: it is an opaque value for it, only has meaning for the driver.
> > >
> > > But the device returns the buffer id when it writes the used descriptor to
> > > the descriptor ring. The "only has meaning for the driver" part has got me
> > > a little confused. Which buffer id is this that the device returns? Is it
> > > related to the buffer id in the available descriptor?
> >
> > In my understanding, buffer id is the element that avail descriptor marks to
> > identify when adding descriptors to table. Device will returns the buffer
> > id in the processed descriptor or the last descriptor in a chain, and write
> > it to the descriptor that used idx refers to (first one in the chain). Then
> > used idx increments.
> >
> > The Packed Virtqueue blog [1] is helpful, but some details in the examples
> > are making me confused.
> >
> > Q1.
> > In the last step of the two-entries descriptor table example, it says both
> > buffers #0 and #1 are available for the device. I understand descriptor[0]
> > is available and descriptor[1] is not, but there is no ID #0 now. So does
> > the device got buffer #0 by notification beforehand? If so, does it mean
> > buffer #0 will be lost when notifications are disabled?
> >
>

I guess you mean the table labeled "Figure: Full two-entries descriptor table".

Take into account that the descriptor table is not the state of all
the descriptors. That information must be maintained by the device and
the driver internally.

The descriptor table is used as a circular buffer, where one part is
writable by the driver and the other part is writable by the device.
For the device to override the descriptor table entry where descriptor
id 0 used to be does not mean that the descriptor id 0 is used. It
just means that the device communicates to the driver that descriptor
1 is used, and both sides need to keep the descriptor state
coherently.

> I too have a similar question and understanding the relation between buffer
> ids in the used and available descriptors might give more insight into this. For
> available descriptors, the buffer id is used to associate descriptors with a
> particular buffer. I am still not very sure about ids in used descriptors.
>
> Regarding Q1, both buffers #0 and #1 are available. In the mentioned figure,
> both descriptor[0] and descriptor[1] are available. This figure follows the figure
> with the caption "Using first buffer out of order". So in the first figure the device
> reads buffer #1 and writes the used descriptor but it still has buffer #0 to read.
> That still belongs to the device while buffer #1 can now be handled by the driver
> once again. So in the next figure, the driver makes buffer #1 available again. The
> device can still read buffer #0 from the previous batch of available descriptors.
>
> Based on what I have understood, the driver can't touch the descriptor
> corresponding to buffer #0 until the device acknowledges it. I did find the
> figure a little confusing as well. I think once the meaning of buffer id is clear
> from the driver's and device's perspective, it'll be easier to understand the
> figure.
>

I think you got it right. Please let me know if you have further questions.

> I am also not very sure about what happens when notifications are disabled.
> I'll have to read up on that again. But I believe the driver still won't be able to
> touch #0 until the device uses it.
>

If one side disables notification it needs to check the indexes or the
flags by its own means: Timers, read the memory in a busy loop, etc.

Sorry for the late reply!

Thanks!

> I think going through the source should better explain these concepts.
>
> Thanks,
> Sahil
>
>
>