Messages in this thread Patch in this message | | | Subject | Re: [PATCH 1/2] vdpa/mlx5: Avoid unnecessary query virtqueue | From | Jason Wang <> | Date | Wed, 3 Feb 2021 13:16:14 +0800 |
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On 2021/2/3 上午1:54, Si-Wei Liu wrote: > On Tue, Feb 2, 2021 at 1:23 AM Eli Cohen <elic@nvidia.com> wrote: >> On Tue, Feb 02, 2021 at 12:38:51AM -0800, Si-Wei Liu wrote: >>> Thanks Eli and Jason for clarifications. See inline. >>> >>> On Mon, Feb 1, 2021 at 11:06 PM Eli Cohen <elic@nvidia.com> wrote: >>>> On Tue, Feb 02, 2021 at 02:02:25PM +0800, Jason Wang wrote: >>>>> On 2021/2/2 下午12:15, Si-Wei Liu wrote: >>>>>> On Mon, Feb 1, 2021 at 7:13 PM Jason Wang <jasowang@redhat.com> wrote: >>>>>>> On 2021/2/2 上午3:17, Si-Wei Liu wrote: >>>>>>>> On Mon, Feb 1, 2021 at 10:51 AM Si-Wei Liu <siwliu.kernel@gmail.com> wrote: >>>>>>>>> On Thu, Jan 28, 2021 at 5:46 AM Eli Cohen <elic@nvidia.com> wrote: >>>>>>>>>> suspend_vq should only suspend the VQ on not save the current available >>>>>>>>>> index. This is done when a change of map occurs when the driver calls >>>>>>>>>> save_channel_info(). >>>>>>>>> Hmmm, suspend_vq() is also called by teardown_vq(), the latter of >>>>>>>>> which doesn't save the available index as save_channel_info() doesn't >>>>>>>>> get called in that path at all. How does it handle the case that >>>>>>>>> aget_vq_state() is called from userspace (e.g. QEMU) while the >>>>>>>>> hardware VQ object was torn down, but userspace still wants to access >>>>>>>>> the queue index? >>>>>>>>> >>>>>>>>> Refer to https://lore.kernel.org/netdev/1601583511-15138-1-git-send-email-si-wei.liu@oracle.com/ >>>>>>>>> >>>>>>>>> vhost VQ 0 ring restore failed: -1: Resource temporarily unavailable (11) >>>>>>>>> vhost VQ 1 ring restore failed: -1: Resource temporarily unavailable (11) >>>>>>>>> >>>>>>>>> QEMU will complain with the above warning while VM is being rebooted >>>>>>>>> or shut down. >>>>>>>>> >>>>>>>>> Looks to me either the kernel driver should cover this requirement, or >>>>>>>>> the userspace has to bear the burden in saving the index and not call >>>>>>>>> into kernel if VQ is destroyed. >>>>>>>> Actually, the userspace doesn't have the insights whether virt queue >>>>>>>> will be destroyed if just changing the device status via set_status(). >>>>>>>> Looking at other vdpa driver in tree i.e. ifcvf it doesn't behave like >>>>>>>> so. Hence this still looks to me to be Mellanox specifics and >>>>>>>> mlx5_vdpa implementation detail that shouldn't expose to userspace. >>>>>>> So I think we can simply drop this patch? >>>>>> Yep, I think so. To be honest I don't know why it has anything to do >>>>>> with the memory hotplug issue. >>>>> >>>>> Eli may know more, my understanding is that, during memory hotplut, qemu >>>>> need to propagate new memory mappings via set_map(). For mellanox, it means >>>>> it needs to rebuild memory keys, so the virtqueue needs to be suspended. >>>>> >>>> I think Siwei was asking why the first patch was related to the hotplug >>>> issue. >>> I was thinking how consistency is assured when saving/restoring this >>> h/w avail_index against the one in the virtq memory, particularly in >>> the region_add/.region_del() context (e.g. the hotplug case). Problem >>> is I don't see explicit memory barrier when guest thread updates the >>> avail_index, how does the device make sure the h/w avail_index is >>> uptodate while guest may race with updating the virtq's avail_index in >>> the mean while? Maybe I completely miss something obvious? >> DKIM-Signature: v1; arsa-sha256; crelaxed/relaxed; dnvidia.com; sn1; >> t 12257780; bhHnB0z4VEKwRS3WGY8d836MJgxu5Eln/jbFZlNXVxc08; >> hX-PGP-Universal:Date:From:To:CC:Subject:Message-ID:References: >> MIME-Version:Content-Type:Content-Disposition: >> Content-Transfer-Encoding:In-Reply-To:User-Agent:X-Originating-IP: >> X-ClientProxiedBy; >> bgGmb8+rcn3/rKzKQ/7QzSnghWzZ+FAU0XntsRZYGQ66sFvT7zsYPHogG3LIWNY77t >> wNHPw7GCJrNaH3nEXPbOp0FMOZw4Kv4W7UPuYPobbLeTkvuPAidjB8dM42vz+1X61t >> 9IVQT9X4hnAxRjI5CqZOo41GS4Tl1X+ykGoA+VE80BR/R/+nQ3tXDVULfppzeB+vu3 >> TWnnpaZ2GyoNyPlMiyVRkHdXzDVgA4uQHxwHn7otGK5J4lzyu8KrFyQtiP+f6hfu5v >> crJkYS8e9A+rfzUmKWuyHcKcmhPhAVJ4XdpzZcDXXlMHVxG7nR1o88xttC6D1+oNIP >> 9xHI3DkNBpEuA >> If you're asking about syncronization upon hot plug of memory, the >> hardware always goes to read the available index from memory when a new >> hardware object is associted with a virtqueue. You can argue then that >> you don't need to restore the available index and you may be right but >> this is the currect inteface to the firmware. >> >> >> If you're asking on generally how sync is assured when the guest updates >> the available index, can you please send a pointer to the code where you >> see the update without a memory barrier? > This is a snippet of virtqueue_add_split() where avail_index gets > updated by guest: > > /* Put entry in available array (but don't update avail->idx until they > * do sync). */ > avail = vq->split.avail_idx_shadow & (vq->split.vring.num - 1); > vq->split.vring.avail->ring[avail] = cpu_to_virtio16(_vq->vdev, head); > > /* Descriptors and available array need to be set before we expose the > * new available array entries. */ > virtio_wmb(vq->weak_barriers); > vq->split.avail_idx_shadow++; > vq->split.vring.avail->idx = cpu_to_virtio16(_vq->vdev, > vq->split.avail_idx_shadow); > vq->num_added++; > > There's memory barrier to make sure the update to descriptor and > available ring is seen before writing to the avail->idx, but there > seems no gurantee that this update would flush to the memory > immmedately either before or after the mlx5-vdpa is suspened and gets > the old avail_index value stashed somewhere. In this case, how does > the hardware ensure the consistency between the guest virtio and host > mlx5-vdpa? Or, it completly relies on guest to update the avail_index > once the next buffer is available, so that the index will be in sync > again?
I'm not sure I get the question but notice that the driver should check and notify virtqueue when device want a notification. So there's a virtio_wmb() e.g in:
static bool virtqueue_kick_prepare_split(struct virtqueue *_vq) { struct vring_virtqueue *vq = to_vvq(_vq); u16 new, old; bool needs_kick;
START_USE(vq); /* We need to expose available array entries before checking avail * event. */ virtio_mb(vq->weak_barries);
old = vq->split.avail_idx_shadow - vq->num_added; new = vq->split.avail_idx_shadow; vq->num_added = 0;
(See the comment above virtio_mb()). So the avail idx is guaranteed to be committed to memroy(cache hierarchy) before the check of notification. I think we sync through this.
Thanks
> > Thanks, > -Siwei > >>> Thanks, >>> -Siwei >>> >>>> But you're correct. When memory is added, I get a new memory map. This >>>> requires me to build a new memory key object which covers the new memory >>>> map. Since the virtqueue objects are referencing this memory key, I need >>>> to destroy them and build new ones referncing the new memory key. >>>> >>>>> Thanks >>>>> >>>>> >>>>>> -Siwei >>>>>> >>>>>>> Thanks >>>>>>> >>>>>>> >>>>>>>>> -Siwei >>>>>>>>> >>>>>>>>> >>>>>>>>>> Signed-off-by: Eli Cohen <elic@nvidia.com> >>>>>>>>>> --- >>>>>>>>>> drivers/vdpa/mlx5/net/mlx5_vnet.c | 8 -------- >>>>>>>>>> 1 file changed, 8 deletions(-) >>>>>>>>>> >>>>>>>>>> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c >>>>>>>>>> index 88dde3455bfd..549ded074ff3 100644 >>>>>>>>>> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c >>>>>>>>>> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c >>>>>>>>>> @@ -1148,8 +1148,6 @@ static int setup_vq(struct mlx5_vdpa_net *ndev, struct mlx5_vdpa_virtqueue *mvq) >>>>>>>>>> >>>>>>>>>> static void suspend_vq(struct mlx5_vdpa_net *ndev, struct mlx5_vdpa_virtqueue *mvq) >>>>>>>>>> { >>>>>>>>>> - struct mlx5_virtq_attr attr; >>>>>>>>>> - >>>>>>>>>> if (!mvq->initialized) >>>>>>>>>> return; >>>>>>>>>> >>>>>>>>>> @@ -1158,12 +1156,6 @@ static void suspend_vq(struct mlx5_vdpa_net *ndev, struct mlx5_vdpa_virtqueue *m >>>>>>>>>> >>>>>>>>>> if (modify_virtqueue(ndev, mvq, MLX5_VIRTIO_NET_Q_OBJECT_STATE_SUSPEND)) >>>>>>>>>> mlx5_vdpa_warn(&ndev->mvdev, "modify to suspend failed\n"); >>>>>>>>>> - >>>>>>>>>> - if (query_virtqueue(ndev, mvq, &attr)) { >>>>>>>>>> - mlx5_vdpa_warn(&ndev->mvdev, "failed to query virtqueue\n"); >>>>>>>>>> - return; >>>>>>>>>> - } >>>>>>>>>> - mvq->avail_idx = attr.available_index; >>>>>>>>>> } >>>>>>>>>> >>>>>>>>>> static void suspend_vqs(struct mlx5_vdpa_net *ndev) >>>>>>>>>> -- >>>>>>>>>> 2.29.2 >>>>>>>>>>
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