On 06.10.23 15:55, Hanna Czenczek wrote:
On 06.10.23 10:49, Michael S. Tsirkin wrote:
On Fri, Oct 06, 2023 at 09:53:53AM +0200, Hanna Czenczek wrote:
On 05.10.23 19:38, Stefan Hajnoczi wrote:
On Wed, Oct 04, 2023 at 02:58:58PM +0200, Hanna Czenczek wrote:
[...]
``VHOST_USER_GET_VRING_BASE``
:id: 11
:equivalent ioctl: ``VHOST_USER_GET_VRING_BASE``
:request payload: vring state description
- :reply payload: vring state description
+ :reply payload: vring descriptor index/indices
+
+ Stops the vring and returns the current descriptor index or
indices:
+
+ * For a split virtqueue, returns only the 16-bit next descriptor
+ index in the *Available Ring*. The index in the *Used Ring* is
+ controlled by the guest driver and can be read from the vring
I find "is controlled by the guest driver" confusing. The device
writes
the Used Ring index. The driver only reads it. The device is the
active
party here.
Er, good point. That breaks the whole reasoning. Then I don’t
understand
why we do get/set the available ring index and not the used ring
index. Do
you know why?
It's simple. used ring index in memory is controlled by the device and
reflects device state.
Exactly, it’s device state, that’s why I thought the front-end needs
to ensure its read and restored around the reset we currently have in
vhost_dev_stop()/start().
device can just read it back to restore.
I find it strange that the device is supposed to read its own state
from memory.
available ring index in memory is controlled by driver and does
not reflect device state.
Why can’t the device read the available index from memory? That value
is put into memory by the driver precisely so the device can read it
from there.
Ah, wait, is the idea that the device may have an internal available
index counter that reflects what descriptor it has already fetched? I.e.
this index will lag behind the one in memory, and the difference are new
descriptors that the device still needs to read? If that internal
counter is the index that’s get/set here, then yes, that makes a lot of
sense.
Hanna
