* vhost-user protocol feature negotiation
@ 2020-08-05 15:13 Alyssa Ross
2020-08-05 22:26 ` Michael S. Tsirkin
0 siblings, 1 reply; 6+ messages in thread
From: Alyssa Ross @ 2020-08-05 15:13 UTC (permalink / raw)
To: Michael S. Tsirkin, qemu-devel
Quoting from the definition of VHOST_USER_SET_PROTOCOL_FEATURES in
vhost-user.rst:
> Only legal if feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` is present in
> ``VHOST_USER_GET_FEATURES``.
>
> .. Note::
> Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must support
> this message even before ``VHOST_USER_SET_FEATURES`` was called.
To me, this could mean either of two things:
(1) If VHOST_USER_F_PROTOCOL_FEATURES hasn't been set, upon receiving
VHOST_USER_SET_PROTOCOL_FEATURES, a backend should enable the
protocol features immediately.
(2) If VHOST_USER_F_PROTOCOL_FEATURES hasn't been set, upon receiving
VHOST_USER_SET_PROTOCOL_FEATURES, a backend should store those
feature bits, but not actually consider them to be enabled until
after VHOST_USER_SET_FEATURES has been received (presumably
containing VHOST_USER_F_PROTOCOL_FEATURES).
The reason I bring this up is that QEMU appears to interpret it as (1),
while the vhost-user-net backend in Intel's cloud-hypervisor[1]
interprets it as (2). So I'm looking for a clarification.
[1]: https://github.com/cloud-hypervisor/cloud-hypervisor
Thanks in advance.
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: vhost-user protocol feature negotiation 2020-08-05 15:13 vhost-user protocol feature negotiation Alyssa Ross @ 2020-08-05 22:26 ` Michael S. Tsirkin 2020-08-06 8:59 ` Alyssa Ross 0 siblings, 1 reply; 6+ messages in thread From: Michael S. Tsirkin @ 2020-08-05 22:26 UTC (permalink / raw) To: Alyssa Ross; +Cc: qemu-devel On Wed, Aug 05, 2020 at 03:13:06PM +0000, Alyssa Ross wrote: > Quoting from the definition of VHOST_USER_SET_PROTOCOL_FEATURES in > vhost-user.rst: > > > Only legal if feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` is present in > > ``VHOST_USER_GET_FEATURES``. > > > > .. Note:: > > Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must support > > this message even before ``VHOST_USER_SET_FEATURES`` was called. > > To me, this could mean either of two things: > > (1) If VHOST_USER_F_PROTOCOL_FEATURES hasn't been set, upon receiving > VHOST_USER_SET_PROTOCOL_FEATURES, a backend should enable the > protocol features immediately. > > (2) If VHOST_USER_F_PROTOCOL_FEATURES hasn't been set, upon receiving > VHOST_USER_SET_PROTOCOL_FEATURES, a backend should store those > feature bits, but not actually consider them to be enabled until > after VHOST_USER_SET_FEATURES has been received (presumably > containing VHOST_USER_F_PROTOCOL_FEATURES). > > The reason I bring this up is that QEMU appears to interpret it as (1), > while the vhost-user-net backend in Intel's cloud-hypervisor[1] > interprets it as (2). So I'm looking for a clarification. > > [1]: https://github.com/cloud-hypervisor/cloud-hypervisor > > Thanks in advance. IMHO the intent was this: VHOST_USER_F_PROTOCOL_FEATURES bit in VHOST_USER_GET_FEATURES means that qemu can send VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES. With most feature bits in VHOST_USER_GET_FEATURES, the specific functionality needs to only be enabled after VHOST_USER_SET_FEATURES. However, this is for functionality dealing with guest activity. VHOST_USER_SET_PROTOCOL_FEATURES has nothing to do with guest directly, it's about negotiation between qemu and backend: it is only in VHOST_USER_GET_FEATURES for the reason that this is the only message (very) old backends reported. Thus, the backend should not check whether VHOST_USER_SET_FEATURES sets VHOST_USER_F_PROTOCOL_FEATURES, instead it should simply always be ready to receive VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES. Backend that isn't always ready to handle VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES should not set VHOST_USER_F_PROTOCOL_FEATURES in VHOST_USER_GET_FEATURES. This appears to be closer to (1), but if qemu can't distinguish then we don't care, right? For example, VHOST_USER_PROTOCOL_F_REPLY_ACK enables acks on arbitrary messages. Does the backend in question ignore the affected bit until SET_FEATURES? If yes won't this make qemu hang? How would you suggest clarifying the wording? Thanks, -- MST ^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: vhost-user protocol feature negotiation 2020-08-05 22:26 ` Michael S. Tsirkin @ 2020-08-06 8:59 ` Alyssa Ross 2020-08-06 9:49 ` Michael S. Tsirkin 0 siblings, 1 reply; 6+ messages in thread From: Alyssa Ross @ 2020-08-06 8:59 UTC (permalink / raw) To: Michael S. Tsirkin; +Cc: qemu-devel "Michael S. Tsirkin" <mst@redhat.com> writes: > On Wed, Aug 05, 2020 at 03:13:06PM +0000, Alyssa Ross wrote: >> Quoting from the definition of VHOST_USER_SET_PROTOCOL_FEATURES in >> vhost-user.rst: >> >> > Only legal if feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` is present in >> > ``VHOST_USER_GET_FEATURES``. >> > >> > .. Note:: >> > Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must support >> > this message even before ``VHOST_USER_SET_FEATURES`` was called. >> >> To me, this could mean either of two things: >> >> (1) If VHOST_USER_F_PROTOCOL_FEATURES hasn't been set, upon receiving >> VHOST_USER_SET_PROTOCOL_FEATURES, a backend should enable the >> protocol features immediately. >> >> (2) If VHOST_USER_F_PROTOCOL_FEATURES hasn't been set, upon receiving >> VHOST_USER_SET_PROTOCOL_FEATURES, a backend should store those >> feature bits, but not actually consider them to be enabled until >> after VHOST_USER_SET_FEATURES has been received (presumably >> containing VHOST_USER_F_PROTOCOL_FEATURES). >> >> The reason I bring this up is that QEMU appears to interpret it as (1), >> while the vhost-user-net backend in Intel's cloud-hypervisor[1] >> interprets it as (2). So I'm looking for a clarification. >> >> [1]: https://github.com/cloud-hypervisor/cloud-hypervisor >> >> Thanks in advance. > > > IMHO the intent was this: VHOST_USER_F_PROTOCOL_FEATURES bit in > VHOST_USER_GET_FEATURES means that qemu can send > VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES. > > With most feature bits in VHOST_USER_GET_FEATURES, the > specific functionality needs to only be enabled after > VHOST_USER_SET_FEATURES. > > However, this is for functionality dealing with guest activity. > VHOST_USER_SET_PROTOCOL_FEATURES has nothing to do with guest directly, > it's about negotiation between qemu and backend: it is only in > VHOST_USER_GET_FEATURES for the reason that this is the only message > (very) old backends reported. Thus, the backend should not check > whether VHOST_USER_SET_FEATURES sets VHOST_USER_F_PROTOCOL_FEATURES, > instead it should simply always be ready to receive > VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES. > > Backend that isn't always ready to handle > VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES > should not set VHOST_USER_F_PROTOCOL_FEATURES in > VHOST_USER_GET_FEATURES. Thanks for the explanation. That matches what I had in mind with (1). > This appears to be closer to (1), but if qemu can't distinguish > then we don't care, right? For example, VHOST_USER_PROTOCOL_F_REPLY_ACK > enables acks on arbitrary messages. Does the backend in question > ignore the affected bit until SET_FEATURES? If yes won't this > make qemu hang? Yes. That was my motivation for asking what the correct behaviour was, so that I could fix the incorrect one. :) I suspect that up to this point, the cloud-hypervisor vhost-user-net backend has only been used with cloud-hypervisor, and so this incompatibilty with QEMU was not noticed. > How would you suggest clarifying the wording? Do you think this communicates everything required? --- diff --git i/docs/interop/vhost-user.rst w/docs/interop/vhost-user.rst index 10e3e3475e..72724d292a 100644 --- i/docs/interop/vhost-user.rst +++ w/docs/interop/vhost-user.rst @@ -854,9 +854,8 @@ Master message types ``VHOST_USER_GET_FEATURES``. .. Note:: - Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must - support this message even before ``VHOST_USER_SET_FEATURES`` was - called. + ``VHOST_USER_F_PROTOCOL_FEATURES`` does not need to be acknowledged + with ``VHOST_USER_SET_FEATURES``. ``VHOST_USER_SET_PROTOCOL_FEATURES`` :id: 16 @@ -869,8 +868,8 @@ Master message types ``VHOST_USER_GET_FEATURES``. .. Note:: - Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must support - this message even before ``VHOST_USER_SET_FEATURES`` was called. + ``VHOST_USER_F_PROTOCOL_FEATURES`` does not need to be acknowledged + with ``VHOST_USER_SET_FEATURES``. ``VHOST_USER_SET_OWNER`` :id: 3 ^ permalink raw reply related [flat|nested] 6+ messages in thread
* Re: vhost-user protocol feature negotiation 2020-08-06 8:59 ` Alyssa Ross @ 2020-08-06 9:49 ` Michael S. Tsirkin 2020-08-06 11:24 ` Alyssa Ross 0 siblings, 1 reply; 6+ messages in thread From: Michael S. Tsirkin @ 2020-08-06 9:49 UTC (permalink / raw) To: Alyssa Ross; +Cc: qemu-devel On Thu, Aug 06, 2020 at 08:59:09AM +0000, Alyssa Ross wrote: > "Michael S. Tsirkin" <mst@redhat.com> writes: > > > On Wed, Aug 05, 2020 at 03:13:06PM +0000, Alyssa Ross wrote: > >> Quoting from the definition of VHOST_USER_SET_PROTOCOL_FEATURES in > >> vhost-user.rst: > >> > >> > Only legal if feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` is present in > >> > ``VHOST_USER_GET_FEATURES``. > >> > > >> > .. Note:: > >> > Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must support > >> > this message even before ``VHOST_USER_SET_FEATURES`` was called. > >> > >> To me, this could mean either of two things: > >> > >> (1) If VHOST_USER_F_PROTOCOL_FEATURES hasn't been set, upon receiving > >> VHOST_USER_SET_PROTOCOL_FEATURES, a backend should enable the > >> protocol features immediately. > >> > >> (2) If VHOST_USER_F_PROTOCOL_FEATURES hasn't been set, upon receiving > >> VHOST_USER_SET_PROTOCOL_FEATURES, a backend should store those > >> feature bits, but not actually consider them to be enabled until > >> after VHOST_USER_SET_FEATURES has been received (presumably > >> containing VHOST_USER_F_PROTOCOL_FEATURES). > >> > >> The reason I bring this up is that QEMU appears to interpret it as (1), > >> while the vhost-user-net backend in Intel's cloud-hypervisor[1] > >> interprets it as (2). So I'm looking for a clarification. > >> > >> [1]: https://github.com/cloud-hypervisor/cloud-hypervisor > >> > >> Thanks in advance. > > > > > > IMHO the intent was this: VHOST_USER_F_PROTOCOL_FEATURES bit in > > VHOST_USER_GET_FEATURES means that qemu can send > > VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES. > > > > With most feature bits in VHOST_USER_GET_FEATURES, the > > specific functionality needs to only be enabled after > > VHOST_USER_SET_FEATURES. > > > > However, this is for functionality dealing with guest activity. > > VHOST_USER_SET_PROTOCOL_FEATURES has nothing to do with guest directly, > > it's about negotiation between qemu and backend: it is only in > > VHOST_USER_GET_FEATURES for the reason that this is the only message > > (very) old backends reported. Thus, the backend should not check > > whether VHOST_USER_SET_FEATURES sets VHOST_USER_F_PROTOCOL_FEATURES, > > instead it should simply always be ready to receive > > VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES. > > > > Backend that isn't always ready to handle > > VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES > > should not set VHOST_USER_F_PROTOCOL_FEATURES in > > VHOST_USER_GET_FEATURES. > > Thanks for the explanation. That matches what I had in mind with (1). > > > This appears to be closer to (1), but if qemu can't distinguish > > then we don't care, right? For example, VHOST_USER_PROTOCOL_F_REPLY_ACK > > enables acks on arbitrary messages. Does the backend in question > > ignore the affected bit until SET_FEATURES? If yes won't this > > make qemu hang? > > Yes. That was my motivation for asking what the correct behaviour was, > so that I could fix the incorrect one. :) I suspect that up to this point, > the cloud-hypervisor vhost-user-net backend has only been used with > cloud-hypervisor, and so this incompatibilty with QEMU was not noticed. > > > How would you suggest clarifying the wording? > > Do you think this communicates everything required? > > --- > diff --git i/docs/interop/vhost-user.rst w/docs/interop/vhost-user.rst > index 10e3e3475e..72724d292a 100644 > --- i/docs/interop/vhost-user.rst > +++ w/docs/interop/vhost-user.rst > @@ -854,9 +854,8 @@ Master message types > ``VHOST_USER_GET_FEATURES``. > > .. Note:: > - Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must > - support this message even before ``VHOST_USER_SET_FEATURES`` was > - called. > + ``VHOST_USER_F_PROTOCOL_FEATURES`` does not need to be acknowledged > + with ``VHOST_USER_SET_FEATURES``. > > ``VHOST_USER_SET_PROTOCOL_FEATURES`` > :id: 16 Hmm I find this confusing. I think it's a good policy to ask qemu to acknowledge it. It's just that the client should not wait for VHOST_USER_SET_FEATURES before handling VHOST_USER_SET_PROTOCOL_FEATURES or VHOST_USER_GET_PROTOCOL_FEATURES. > @@ -869,8 +868,8 @@ Master message types > ``VHOST_USER_GET_FEATURES``. > > .. Note:: > - Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must support > - this message even before ``VHOST_USER_SET_FEATURES`` was called. > + ``VHOST_USER_F_PROTOCOL_FEATURES`` does not need to be acknowledged > + with ``VHOST_USER_SET_FEATURES``. > > ``VHOST_USER_SET_OWNER`` > :id: 3 ^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: vhost-user protocol feature negotiation 2020-08-06 9:49 ` Michael S. Tsirkin @ 2020-08-06 11:24 ` Alyssa Ross 2020-08-06 12:35 ` Michael S. Tsirkin 0 siblings, 1 reply; 6+ messages in thread From: Alyssa Ross @ 2020-08-06 11:24 UTC (permalink / raw) To: Michael S. Tsirkin; +Cc: qemu-devel "Michael S. Tsirkin" <mst@redhat.com> writes: > On Thu, Aug 06, 2020 at 08:59:09AM +0000, Alyssa Ross wrote: >> "Michael S. Tsirkin" <mst@redhat.com> writes: >> >> > On Wed, Aug 05, 2020 at 03:13:06PM +0000, Alyssa Ross wrote: >> >> Quoting from the definition of VHOST_USER_SET_PROTOCOL_FEATURES in >> >> vhost-user.rst: >> >> >> >> > Only legal if feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` is present in >> >> > ``VHOST_USER_GET_FEATURES``. >> >> > >> >> > .. Note:: >> >> > Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must support >> >> > this message even before ``VHOST_USER_SET_FEATURES`` was called. >> >> >> >> To me, this could mean either of two things: >> >> >> >> (1) If VHOST_USER_F_PROTOCOL_FEATURES hasn't been set, upon receiving >> >> VHOST_USER_SET_PROTOCOL_FEATURES, a backend should enable the >> >> protocol features immediately. >> >> >> >> (2) If VHOST_USER_F_PROTOCOL_FEATURES hasn't been set, upon receiving >> >> VHOST_USER_SET_PROTOCOL_FEATURES, a backend should store those >> >> feature bits, but not actually consider them to be enabled until >> >> after VHOST_USER_SET_FEATURES has been received (presumably >> >> containing VHOST_USER_F_PROTOCOL_FEATURES). >> >> >> >> The reason I bring this up is that QEMU appears to interpret it as (1), >> >> while the vhost-user-net backend in Intel's cloud-hypervisor[1] >> >> interprets it as (2). So I'm looking for a clarification. >> >> >> >> [1]: https://github.com/cloud-hypervisor/cloud-hypervisor >> >> >> >> Thanks in advance. >> > >> > >> > IMHO the intent was this: VHOST_USER_F_PROTOCOL_FEATURES bit in >> > VHOST_USER_GET_FEATURES means that qemu can send >> > VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES. >> > >> > With most feature bits in VHOST_USER_GET_FEATURES, the >> > specific functionality needs to only be enabled after >> > VHOST_USER_SET_FEATURES. >> > >> > However, this is for functionality dealing with guest activity. >> > VHOST_USER_SET_PROTOCOL_FEATURES has nothing to do with guest directly, >> > it's about negotiation between qemu and backend: it is only in >> > VHOST_USER_GET_FEATURES for the reason that this is the only message >> > (very) old backends reported. Thus, the backend should not check >> > whether VHOST_USER_SET_FEATURES sets VHOST_USER_F_PROTOCOL_FEATURES, >> > instead it should simply always be ready to receive >> > VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES. >> > >> > Backend that isn't always ready to handle >> > VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES >> > should not set VHOST_USER_F_PROTOCOL_FEATURES in >> > VHOST_USER_GET_FEATURES. >> >> Thanks for the explanation. That matches what I had in mind with (1). >> >> > This appears to be closer to (1), but if qemu can't distinguish >> > then we don't care, right? For example, VHOST_USER_PROTOCOL_F_REPLY_ACK >> > enables acks on arbitrary messages. Does the backend in question >> > ignore the affected bit until SET_FEATURES? If yes won't this >> > make qemu hang? >> >> Yes. That was my motivation for asking what the correct behaviour was, >> so that I could fix the incorrect one. :) I suspect that up to this point, >> the cloud-hypervisor vhost-user-net backend has only been used with >> cloud-hypervisor, and so this incompatibilty with QEMU was not noticed. >> >> > How would you suggest clarifying the wording? >> >> Do you think this communicates everything required? >> >> --- >> diff --git i/docs/interop/vhost-user.rst w/docs/interop/vhost-user.rst >> index 10e3e3475e..72724d292a 100644 >> --- i/docs/interop/vhost-user.rst >> +++ w/docs/interop/vhost-user.rst >> @@ -854,9 +854,8 @@ Master message types >> ``VHOST_USER_GET_FEATURES``. >> >> .. Note:: >> - Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must >> - support this message even before ``VHOST_USER_SET_FEATURES`` was >> - called. >> + ``VHOST_USER_F_PROTOCOL_FEATURES`` does not need to be acknowledged >> + with ``VHOST_USER_SET_FEATURES``. >> >> ``VHOST_USER_SET_PROTOCOL_FEATURES`` >> :id: 16 > > Hmm I find this confusing. I think it's a good policy to ask qemu to > acknowledge it. It's just that the client should not wait for > VHOST_USER_SET_FEATURES before handling VHOST_USER_SET_PROTOCOL_FEATURES > or VHOST_USER_GET_PROTOCOL_FEATURES. To me, it's confusing that a frontend is expected to ack VHOST_USER_F_PROTOCOL_FEATURES even though the ack can't have any effect (because VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES both have to work even if the ack hasn't been received yet). But, if the frontend is supposed to ack anyway, how about: Signed-off-by: Alyssa Ross <hi@alysas.is> --- diff --git i/docs/interop/vhost-user.rst w/docs/interop/vhost-user.rst index 10e3e3475e..bc78c9947f 100644 --- i/docs/interop/vhost-user.rst +++ w/docs/interop/vhost-user.rst @@ -854,9 +854,9 @@ Master message types ``VHOST_USER_GET_FEATURES``. .. Note:: - Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must - support this message even before ``VHOST_USER_SET_FEATURES`` was - called. + While QEMU should acknowledge ``VHOST_USER_F_PROTOCOL_FEATURES``, a + backend must allow ``VHOST_USER_GET_PROTOCOL_FEATURES`` even if + ``VHOST_USER_F_PROTOCOL_FEATURES`` has not been acknowledged yet. ``VHOST_USER_SET_PROTOCOL_FEATURES`` :id: 16 @@ -869,8 +869,12 @@ Master message types ``VHOST_USER_GET_FEATURES``. .. Note:: - Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must support - this message even before ``VHOST_USER_SET_FEATURES`` was called. + While QEMU should acknowledge ``VHOST_USER_F_PROTOCOL_FEATURES``, a + backend must allow ``VHOST_USER_SET_PROTOCOL_FEATURES`` even if + ``VHOST_USER_F_PROTOCOL_FEATURES`` has not been acknowledged yet. + The backend must not wait for ``VHOST_USER_SET_FEATURES`` before + enabling protocol features requested with + ``VHOST_USER_SET_PROTOCOL_FEATURES``. ``VHOST_USER_SET_OWNER`` :id: 3 ^ permalink raw reply related [flat|nested] 6+ messages in thread
* Re: vhost-user protocol feature negotiation 2020-08-06 11:24 ` Alyssa Ross @ 2020-08-06 12:35 ` Michael S. Tsirkin 0 siblings, 0 replies; 6+ messages in thread From: Michael S. Tsirkin @ 2020-08-06 12:35 UTC (permalink / raw) To: Alyssa Ross; +Cc: qemu-devel On Thu, Aug 06, 2020 at 11:24:59AM +0000, Alyssa Ross wrote: > "Michael S. Tsirkin" <mst@redhat.com> writes: > > > On Thu, Aug 06, 2020 at 08:59:09AM +0000, Alyssa Ross wrote: > >> "Michael S. Tsirkin" <mst@redhat.com> writes: > >> > >> > On Wed, Aug 05, 2020 at 03:13:06PM +0000, Alyssa Ross wrote: > >> >> Quoting from the definition of VHOST_USER_SET_PROTOCOL_FEATURES in > >> >> vhost-user.rst: > >> >> > >> >> > Only legal if feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` is present in > >> >> > ``VHOST_USER_GET_FEATURES``. > >> >> > > >> >> > .. Note:: > >> >> > Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must support > >> >> > this message even before ``VHOST_USER_SET_FEATURES`` was called. > >> >> > >> >> To me, this could mean either of two things: > >> >> > >> >> (1) If VHOST_USER_F_PROTOCOL_FEATURES hasn't been set, upon receiving > >> >> VHOST_USER_SET_PROTOCOL_FEATURES, a backend should enable the > >> >> protocol features immediately. > >> >> > >> >> (2) If VHOST_USER_F_PROTOCOL_FEATURES hasn't been set, upon receiving > >> >> VHOST_USER_SET_PROTOCOL_FEATURES, a backend should store those > >> >> feature bits, but not actually consider them to be enabled until > >> >> after VHOST_USER_SET_FEATURES has been received (presumably > >> >> containing VHOST_USER_F_PROTOCOL_FEATURES). > >> >> > >> >> The reason I bring this up is that QEMU appears to interpret it as (1), > >> >> while the vhost-user-net backend in Intel's cloud-hypervisor[1] > >> >> interprets it as (2). So I'm looking for a clarification. > >> >> > >> >> [1]: https://github.com/cloud-hypervisor/cloud-hypervisor > >> >> > >> >> Thanks in advance. > >> > > >> > > >> > IMHO the intent was this: VHOST_USER_F_PROTOCOL_FEATURES bit in > >> > VHOST_USER_GET_FEATURES means that qemu can send > >> > VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES. > >> > > >> > With most feature bits in VHOST_USER_GET_FEATURES, the > >> > specific functionality needs to only be enabled after > >> > VHOST_USER_SET_FEATURES. > >> > > >> > However, this is for functionality dealing with guest activity. > >> > VHOST_USER_SET_PROTOCOL_FEATURES has nothing to do with guest directly, > >> > it's about negotiation between qemu and backend: it is only in > >> > VHOST_USER_GET_FEATURES for the reason that this is the only message > >> > (very) old backends reported. Thus, the backend should not check > >> > whether VHOST_USER_SET_FEATURES sets VHOST_USER_F_PROTOCOL_FEATURES, > >> > instead it should simply always be ready to receive > >> > VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES. > >> > > >> > Backend that isn't always ready to handle > >> > VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES > >> > should not set VHOST_USER_F_PROTOCOL_FEATURES in > >> > VHOST_USER_GET_FEATURES. > >> > >> Thanks for the explanation. That matches what I had in mind with (1). > >> > >> > This appears to be closer to (1), but if qemu can't distinguish > >> > then we don't care, right? For example, VHOST_USER_PROTOCOL_F_REPLY_ACK > >> > enables acks on arbitrary messages. Does the backend in question > >> > ignore the affected bit until SET_FEATURES? If yes won't this > >> > make qemu hang? > >> > >> Yes. That was my motivation for asking what the correct behaviour was, > >> so that I could fix the incorrect one. :) I suspect that up to this point, > >> the cloud-hypervisor vhost-user-net backend has only been used with > >> cloud-hypervisor, and so this incompatibilty with QEMU was not noticed. > >> > >> > How would you suggest clarifying the wording? > >> > >> Do you think this communicates everything required? > >> > >> --- > >> diff --git i/docs/interop/vhost-user.rst w/docs/interop/vhost-user.rst > >> index 10e3e3475e..72724d292a 100644 > >> --- i/docs/interop/vhost-user.rst > >> +++ w/docs/interop/vhost-user.rst > >> @@ -854,9 +854,8 @@ Master message types > >> ``VHOST_USER_GET_FEATURES``. > >> > >> .. Note:: > >> - Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must > >> - support this message even before ``VHOST_USER_SET_FEATURES`` was > >> - called. > >> + ``VHOST_USER_F_PROTOCOL_FEATURES`` does not need to be acknowledged > >> + with ``VHOST_USER_SET_FEATURES``. > >> > >> ``VHOST_USER_SET_PROTOCOL_FEATURES`` > >> :id: 16 > > > > Hmm I find this confusing. I think it's a good policy to ask qemu to > > acknowledge it. It's just that the client should not wait for > > VHOST_USER_SET_FEATURES before handling VHOST_USER_SET_PROTOCOL_FEATURES > > or VHOST_USER_GET_PROTOCOL_FEATURES. > > To me, it's confusing that a frontend is expected to ack > VHOST_USER_F_PROTOCOL_FEATURES even though the ack can't have any effect > (because VHOST_USER_GET_PROTOCOL_FEATURES and > VHOST_USER_SET_PROTOCOL_FEATURES both have to work even if the ack > hasn't been received yet). > > But, if the frontend is supposed to ack anyway, how about: > > Signed-off-by: Alyssa Ross <hi@alysas.is> > > --- > diff --git i/docs/interop/vhost-user.rst w/docs/interop/vhost-user.rst > index 10e3e3475e..bc78c9947f 100644 > --- i/docs/interop/vhost-user.rst > +++ w/docs/interop/vhost-user.rst > @@ -854,9 +854,9 @@ Master message types > ``VHOST_USER_GET_FEATURES``. > > .. Note:: > - Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must > - support this message even before ``VHOST_USER_SET_FEATURES`` was > - called. > + While QEMU should acknowledge ``VHOST_USER_F_PROTOCOL_FEATURES``, a > + backend must allow ``VHOST_USER_GET_PROTOCOL_FEATURES`` even if > + ``VHOST_USER_F_PROTOCOL_FEATURES`` has not been acknowledged yet. > > ``VHOST_USER_SET_PROTOCOL_FEATURES`` > :id: 16 > @@ -869,8 +869,12 @@ Master message types > ``VHOST_USER_GET_FEATURES``. > > .. Note:: > - Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must support > - this message even before ``VHOST_USER_SET_FEATURES`` was called. > + While QEMU should acknowledge ``VHOST_USER_F_PROTOCOL_FEATURES``, a > + backend must allow ``VHOST_USER_SET_PROTOCOL_FEATURES`` even if > + ``VHOST_USER_F_PROTOCOL_FEATURES`` has not been acknowledged yet. > + The backend must not wait for ``VHOST_USER_SET_FEATURES`` before > + enabling protocol features requested with > + ``VHOST_USER_SET_PROTOCOL_FEATURES``. > > ``VHOST_USER_SET_OWNER`` > :id: 3 That looks good to me. Pls post a patch on list, preferably after qemu is released. ^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2020-08-06 12:36 UTC | newest] Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2020-08-05 15:13 vhost-user protocol feature negotiation Alyssa Ross 2020-08-05 22:26 ` Michael S. Tsirkin 2020-08-06 8:59 ` Alyssa Ross 2020-08-06 9:49 ` Michael S. Tsirkin 2020-08-06 11:24 ` Alyssa Ross 2020-08-06 12:35 ` Michael S. Tsirkin
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