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Tsirkin" To: Si-Wei Liu Cc: Shannon Nelson , kvm@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Eugenio Perez Martin Subject: Re: [PATCH] vdpa: reject F_ENABLE_AFTER_DRIVER_OK if backend does not support it Message-ID: <20230719182112-mutt-send-email-mst@kernel.org> References: <20230703105022-mutt-send-email-mst@kernel.org> <20230704063646-mutt-send-email-mst@kernel.org> <20230704114159-mutt-send-email-mst@kernel.org> <20230706020603-mutt-send-email-mst@kernel.org> <1fdf73cb-f23e-0c34-f95f-f1bac74332da@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1fdf73cb-f23e-0c34-f95f-f1bac74332da@oracle.com> Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Wed, Jul 19, 2023 at 03:20:03PM -0700, Si-Wei Liu wrote: > > > On 7/5/2023 11:07 PM, Michael S. Tsirkin wrote: > > On Wed, Jul 05, 2023 at 05:07:11PM -0700, Shannon Nelson wrote: > > > On 7/5/23 11:27 AM, Eugenio Perez Martin wrote: > > > > On Wed, Jul 5, 2023 at 9:50 AM Jason Wang wrote: > > > > > On Tue, Jul 4, 2023 at 11:45 PM Michael S. Tsirkin wrote: > > > > > > On Tue, Jul 04, 2023 at 01:36:11PM +0200, Eugenio Perez Martin wrote: > > > > > > > On Tue, Jul 4, 2023 at 12:38 PM Michael S. Tsirkin wrote: > > > > > > > > On Tue, Jul 04, 2023 at 12:25:32PM +0200, Eugenio Perez Martin wrote: > > > > > > > > > On Mon, Jul 3, 2023 at 4:52 PM Michael S. Tsirkin wrote: > > > > > > > > > > On Mon, Jul 03, 2023 at 04:22:18PM +0200, Eugenio Pérez wrote: > > > > > > > > > > > With the current code it is accepted as long as userland send it. > > > > > > > > > > > > > > > > > > > > > > Although userland should not set a feature flag that has not been > > > > > > > > > > > offered to it with VHOST_GET_BACKEND_FEATURES, the current code will not > > > > > > > > > > > complain for it. > > > > > > > > > > > > > > > > > > > > > > Since there is no specific reason for any parent to reject that backend > > > > > > > > > > > feature bit when it has been proposed, let's control it at vdpa frontend > > > > > > > > > > > level. Future patches may move this control to the parent driver. > > > > > > > > > > > > > > > > > > > > > > Fixes: 967800d2d52e ("vdpa: accept VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK backend feature") > > > > > > > > > > > Signed-off-by: Eugenio Pérez > > > > > > > > > > Please do send v3. And again, I don't want to send "after driver ok" hack > > > > > > > > > > upstream at all, I merged it in next just to give it some testing. > > > > > > > > > > We want RING_ACCESS_AFTER_KICK or some such. > > > > > > > > > > > > > > > > > > > Current devices do not support that semantic. > > > > > > > > Which devices specifically access the ring after DRIVER_OK but before > > > > > > > > a kick? > > > The PDS vdpa device can deal with a call to .set_vq_ready after DRIVER_OK is > > > set. And I'm told that our VQ activity should start without a kick. > > > > > > Our vdpa device FW doesn't currently have support for VIRTIO_F_RING_RESET, > > > but I believe it could be added without too much trouble. > > > > > > sln > > > > > OK it seems clear at least in the current version pds needs > > VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK. > > However can we also code up the RING_RESET path as the default? > What's the rationale of making RING_RESET path the default? Noted this is on > a performance critical path (for live migration downtime), did we ever get > consensus from every or most hardware vendors that RING_RESET has lower cost > in terms of latency overall than ENABLE_AFTER_DRIVER_OK? I think (RING)RESET > in general falls on the slow path for hardware, while I assume either > RING_RESET or ENABLE_AFTER_DRIVER_OK doesn't matters much on software backed > vdpa e.g. vp_vdpa. Maybe should make ENABLE_AFTER_DRIVER_OK as the default? > > -Siwei Coming from the spec RING_RESET has clearer semantics. As long as we support it it is not critical which one is the default though. > > Then down the road vendors can choose what to do. > > > > > > > > > > > > > > > > > Previous versions of the QEMU LM series did a spurious kick to start > > > > > > > traffic at the LM destination [1]. When it was proposed, that spurious > > > > > > > kick was removed from the series because to check for descriptors > > > > > > > after driver_ok, even without a kick, was considered work of the > > > > > > > parent driver. > > > > > > > > > > > > > > I'm ok to go back to this spurious kick, but I'm not sure if the hw > > > > > > > will read the ring before the kick actually. I can ask. > > > > > > > > > > > > > > Thanks! > > > > > > > > > > > > > > [1] https://lists.nongnu.org/archive/html/qemu-devel/2023-01/msg02775.html > > > > > > Let's find out. We need to check for ENABLE_AFTER_DRIVER_OK too, no? > > > > > My understanding is [1] assuming ACCESS_AFTER_KICK. This seems > > > > > sub-optimal than assuming ENABLE_AFTER_DRIVER_OK. > > > > > > > > > > But this reminds me one thing, as the thread is going too long, I > > > > > wonder if we simply assume ENABLE_AFTER_DRIVER_OK if RING_RESET is > > > > > supported? > > > > > > > > > The problem with that is that the device needs to support all > > > > RING_RESET, like to be able to change vq address etc after DRIVER_OK. > > > > Not all HW support it. > > > > > > > > We just need the subset of having the dataplane freezed until all CVQ > > > > commands have been consumed. I'm sure current vDPA code already > > > > supports it in some devices, like MLX and PSD. > > > > > > > > Thanks! > > > > > > > > > Thanks > > > > > > > > > > > > > > > > > > > > > > > > > > My plan was to convert > > > > > > > > > it in vp_vdpa if needed, and reuse the current vdpa ops. Sorry if I > > > > > > > > > was not explicit enough. > > > > > > > > > > > > > > > > > > The only solution I can see to that is to trap & emulate in the vdpa > > > > > > > > > (parent?) driver, as talked in virtio-comment. But that complicates > > > > > > > > > the architecture: > > > > > > > > > * Offer VHOST_BACKEND_F_RING_ACCESS_AFTER_KICK > > > > > > > > > * Store vq enable state separately, at > > > > > > > > > vdpa->config->set_vq_ready(true), but not transmit that enable to hw > > > > > > > > > * Store the doorbell state separately, but do not configure it to the > > > > > > > > > device directly. > > > > > > > > > > > > > > > > > > But how to recover if the device cannot configure them at kick time, > > > > > > > > > for example? > > > > > > > > > > > > > > > > > > Maybe we can just fail if the parent driver does not support enabling > > > > > > > > > the vq after DRIVER_OK? That way no new feature flag is needed. > > > > > > > > > > > > > > > > > > Thanks! > > > > > > > > > > > > > > > > > > > > --- > > > > > > > > > > > Sent with Fixes: tag pointing to git.kernel.org/pub/scm/linux/kernel/git/mst > > > > > > > > > > > commit. Please let me know if I should send a v3 of [1] instead. > > > > > > > > > > > > > > > > > > > > > > [1] https://lore.kernel.org/lkml/20230609121244-mutt-send-email-mst@kernel.org/T/ > > > > > > > > > > > --- > > > > > > > > > > > drivers/vhost/vdpa.c | 7 +++++-- > > > > > > > > > > > 1 file changed, 5 insertions(+), 2 deletions(-) > > > > > > > > > > > > > > > > > > > > > > diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c > > > > > > > > > > > index e1abf29fed5b..a7e554352351 100644 > > > > > > > > > > > --- a/drivers/vhost/vdpa.c > > > > > > > > > > > +++ b/drivers/vhost/vdpa.c > > > > > > > > > > > @@ -681,18 +681,21 @@ static long vhost_vdpa_unlocked_ioctl(struct file *filep, > > > > > > > > > > > { > > > > > > > > > > > struct vhost_vdpa *v = filep->private_data; > > > > > > > > > > > struct vhost_dev *d = &v->vdev; > > > > > > > > > > > + const struct vdpa_config_ops *ops = v->vdpa->config; > > > > > > > > > > > void __user *argp = (void __user *)arg; > > > > > > > > > > > u64 __user *featurep = argp; > > > > > > > > > > > - u64 features; > > > > > > > > > > > + u64 features, parent_features = 0; > > > > > > > > > > > long r = 0; > > > > > > > > > > > > > > > > > > > > > > if (cmd == VHOST_SET_BACKEND_FEATURES) { > > > > > > > > > > > if (copy_from_user(&features, featurep, sizeof(features))) > > > > > > > > > > > return -EFAULT; > > > > > > > > > > > + if (ops->get_backend_features) > > > > > > > > > > > + parent_features = ops->get_backend_features(v->vdpa); > > > > > > > > > > > if (features & ~(VHOST_VDPA_BACKEND_FEATURES | > > > > > > > > > > > BIT_ULL(VHOST_BACKEND_F_SUSPEND) | > > > > > > > > > > > BIT_ULL(VHOST_BACKEND_F_RESUME) | > > > > > > > > > > > - BIT_ULL(VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK))) > > > > > > > > > > > + parent_features)) > > > > > > > > > > > return -EOPNOTSUPP; > > > > > > > > > > > if ((features & BIT_ULL(VHOST_BACKEND_F_SUSPEND)) && > > > > > > > > > > > !vhost_vdpa_can_suspend(v)) > > > > > > > > > > > -- > > > > > > > > > > > 2.39.3 > > _______________________________________________ > > Virtualization mailing list > > Virtualization@lists.linux-foundation.org > > https://lists.linuxfoundation.org/mailman/listinfo/virtualization