On 06.10.23 11:26, Michael S. Tsirkin wrote:
On Fri, Oct 06, 2023 at 11:15:55AM +0200, Hanna Czenczek wrote:
On 06.10.23 10:45, Michael S. Tsirkin wrote:
On Fri, Oct 06, 2023 at 09:48:14AM +0200, Hanna Czenczek wrote:
On 05.10.23 19:15, Michael S. Tsirkin wrote:
On Thu, Oct 05, 2023 at 01:08:52PM -0400, Stefan Hajnoczi wrote:
On Wed, Oct 04, 2023 at 02:58:57PM +0200, Hanna Czenczek wrote:
There is no clearly defined purpose for the virtio status byte in
vhost-user: For resetting, we already have RESET_DEVICE; and for virtio
feature negotiation, we have [GS]ET_FEATURES. With the REPLY_ACK
protocol extension, it is possible for SET_FEATURES to return errors
(SET_PROTOCOL_FEATURES may be called before SET_FEATURES).
As for implementations, SET_STATUS is not widely implemented. dpdk does
implement it, but only uses it to signal feature negotiation failure.
While it does log reset requests (SET_STATUS 0) as such, it effectively
ignores them, in contrast to RESET_OWNER (which is deprecated, and today
means the same thing as RESET_DEVICE).
While qemu superficially has support for [GS]ET_STATUS, it does not
forward the guest-set status byte, but instead just makes it up
internally, and actually completely ignores what the back-end returns,
only using it as the template for a subsequent SET_STATUS to add single
bits to it. Notably, after setting FEATURES_OK, it never reads it back
to see whether the flag is still set, which is the only way in which
dpdk uses the status byte.
As-is, no front-end or back-end can rely on the other side handling this
field in a useful manner, and it also provides no practical use over
other mechanisms the vhost-user protocol has, which are more clearly
defined. Deprecate it.
Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
---
docs/interop/vhost-user.rst | 28 +++++++++++++++++++++-------
1 file changed, 21 insertions(+), 7 deletions(-)
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
SET_STATUS is the only way to signal failure to acknowledge FEATURES_OK.
The fact current backends never check errors does not mean they never
will. So no, not applying this.
Can this not be done with REPLY_ACK? I.e., with the following message
order:
1. GET_FEATURES to find out whether VHOST_USER_F_PROTOCOL_FEATURES is
present
2. GET_PROTOCOL_FEATURES to hopefully get VHOST_USER_PROTOCOL_F_REPLY_ACK
3. SET_PROTOCOL_FEATURES to set VHOST_USER_PROTOCOL_F_REPLY_ACK
4. SET_FEATURES with need_reply
If not, the problem is that qemu has sent SET_STATUS 0 for a while when the
vCPUs are stopped, which generally seems to request a device reset. If we
don’t state at least that SET_STATUS 0 is to be ignored, back-ends that will
implement SET_STATUS later may break with at least these qemu versions. But
documenting that a particular use of the status byte is to be ignored would
be really strange.
Hanna
Hmm I guess. Though just following virtio spec seems cleaner to me...
vhost-user reconfigures the state fully on start.
Not the internal device state, though. virtiofsd has internal state, and
other devices like vhost-gpu back-ends would probably, too.
Stefan has recently sent a series
(https://lists.nongnu.org/archive/html/qemu-devel/2023-10/msg00709.html) to
put the reset (RESET_DEVICE) into virtio_reset() (when we really need a
reset).
I really don’t like our current approach with the status byte. Following the
virtio specification to me would mean that the guest directly controls this
byte, which it does not. qemu makes up values as it deems appropriate, and
this includes sending a SET_STATUS 0 when the guest is just paused, i.e.
when the guest really doesn’t want a device reset.
That means that qemu does not treat this as a virtio device field (because
that would mean exposing it to the guest driver), but instead treats it as
part of the vhost(-user) protocol. It doesn’t feel right to me that we use
a virtio-defined feature for communication on the vhost level, i.e. between
front-end and back-end, and not between guest driver and device. I think
all vhost-level protocol features should be fully defined in the vhost-user
specification, which REPLY_ACK is.
Hmm that makes sense. Maybe we should have done what stefan's patch
is doing.
Do look at the original commit that introduced it to understand why
it was added.
I don’t understand why this was added to the stop/cont code, though. If it
is time consuming to make these changes, why are they done every time the VM
is paused
and resumed? It makes sense that this would be done for the initial
configuration (where a reset also wouldn’t hurt), but here it seems wrong.
(To be clear, a reset in the stop/cont code is wrong, because it breaks
stateful devices.)
Also, note the newer commits 6f8be29ec17 and c3716f260bf. The reset as
originally introduced was wrong even for non-stateful devices, because it
occurred before we fetched the state (vring indices) so we could restore it
later. I don’t know how 923b8921d21 was tested, but if the back-end used
for testing implemented SET_STATUS 0 as a reset, it could not have survived
either migration or a stop/cont in general, because the vring indices would
have been reset to 0.
What I’m saying is, 923b8921d21 introduced SET_STATUS calls that broke all
devices that would implement them as per virtio spec, and even today it’s
broken for stateful devices. The mentioned performance issue is likely
real, but we can’t address it by making up SET_STATUS calls that are wrong.
I concede that I didn’t think about DRIVER_OK. Personally, I would do all
final configuration that would happen upon a DRIVER_OK once the first vring
is started (i.e. receives a kick). That has the added benefit of being
asynchronous because it doesn’t block any vhost-user messages (which are
synchronous, and thus block downtime).
Hanna