Re: Questions about virtio DMA operations

[Arnabjyoti Kalita]

Thank you for the clarification, Peter.

> This sounds to me like the wrong layer to do this. My first
> stab at a design for this would be to record and replay
> at the disk block API level.


Yes, I did follow the upstream record-and-replay feature and did try
it. I can see that it has used a different 'blkreplay' driver to
record the completion "IDs". I am not trying to make the design very
generic right now since our eventual goal isn't a 'design correct'
record-replay system. We are using record-replay of network and disk
I/O to eventually achieve something else. And directly using the
upstream record-and-replay feature is a slight impediment for me since
I record in KVM mode and replay in TCG mode. But I get your point. I
might try a more generic design once I have a working system.

Best Regards,
Arnabjyoti Kalita


On Mon, Aug 30, 2021 at 2:39 PM Peter Maydell <peter.maydell@linaro.org> wrote:
>
> On Mon, 30 Aug 2021 at 03:30, Arnabjyoti Kalita
> <akalita@cs.stonybrook.edu> wrote:
> >
> > > That's normal, yes, but you can't guarantee it. The guest can
> > > choose to program it to any physical memory it likes.
> >
> > Is it ? If this is indeterministic, my approach will obviously not
> > work.
>
> It's deterministic to the extent that the guest execution
> is deterministic. But inside QEMU you can't tell what the
> deterministic guest execution will or will not do.
>
> > Is the bounce buffer address determined at initial startup of
> > the guest and does it always stay the same during the guest execution?
>
> The bounce buffer is at a fixed host address, because it
> is the global "static BounceBuffer bounce;" in physmem.c.
>
> > > I'm not clear on what you're trying to do, but this doesn't
> > > sound like the right way to go to me.
> >
> > So I am trying to record and replay virtio block disk I/O events. I
> > plan to record the disk I/O by recording the "used" buffers that the
> > virtio block device puts into the virtqueue after the actual disk I/O
> > is complete.
>
> This sounds to me like the wrong layer to do this. My first
> stab at a design for this would be to record and replay
> at the disk block API level. That then does not need to care
> at all about any of the details of the virtio device implementation
> (and it would work with any block device, eg scsi, as a bonus).
> This is how the upstream record-and-replay feature handles block
> devices.
>
> -- PMM