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| From: | Si-Wei Liu |
| Subject: | Re: [RFC PATCH 00/18] Map memory at destination .load_setup in vDPA-net migration |
| Date: | Tue, 5 Dec 2023 16:36:09 -0800 |
| User-agent: | Mozilla Thunderbird |
On 12/5/2023 6:23 AM, Eugenio Perez Martin wrote:
On Fri, Nov 3, 2023 at 9:19 PM Si-Wei Liu <si-wei.liu@oracle.com> wrote:On 11/2/2023 5:37 AM, Eugenio Perez Martin wrote:On Thu, Nov 2, 2023 at 11:13 AM Si-Wei Liu <si-wei.liu@oracle.com> wrote:On 10/19/2023 7:34 AM, Eugenio Pérez wrote:Current memory operations like pinning may take a lot of time at the destination. Currently they are done after the source of the migration is stopped, and before the workload is resumed at the destination. This is a period where neigher traffic can flow, nor the VM workload can continue (downtime). We can do better as we know the memory layout of the guest RAM at the destination from the moment the migration starts. Moving that operation allows QEMU to communicate the kernel the maps while the workload is still running in the source, so Linux can start mapping them. Ideally, all IOMMU is configured, but if the vDPA parent driver uses on-chip IOMMU and .set_map we're still saving all the pinning time.I get what you want to say, though not sure how pinning is relevant to on-chip IOMMU and .set_map here, essentially pinning is required for all parent vdpa drivers that perform DMA hence don't want VM pages to move around.Basically highlighting that the work done under .set_map is not only pinning, but it is a significant fraction on it. It can be reworded or deleted for sure.Note that further devices setup at the end of the migration may alter the guest memory layout. But same as the previous point, many operations are still done incrementally, like memory pinning, so we're saving time anyway. The first bunch of patches just reorganizes the code, so memory related operation parameters are shared between all vhost_vdpa devices. This is because the destination does not know what vhost_vdpa struct will have the registered listener member, so it is easier to place them in a shared struct rather to keep them in vhost_vdpa struct. Future version may squash or omit these patches.It looks this VhostVDPAShared facility (patch 1-13) is also what I need in my SVQ descriptor group series [*], for which I've built similar construct there. If possible please try to merge this in ASAP. I'll rework my series on top of that. [*] https://github.com/siwliu-kernel/qemu/commit/813518354af5ee8a6e867b2bf7dff3d6004fbcd5I can send it individually, for sure. MST, Jason, can this first part be merged? It doesn't add a lot by itself but it helps pave the way for future changes.If it cannot, it doesn't matter. I can pick it from here and get my series posted with your patches 1-13 applied upfront. This should work, I think?Only tested with vdpa_sim. I'm sending this before full benchmark, as some work like [1] can be based on it, and Si-Wei agreed on benchmark this series with his experience.Haven't done the full benchmark compared to pre-map at destination yet, though an observation is that the destination QEMU seems very easy to get stuck for very long time while in mid of pinning pages. During this period, any client doing read-only QMP query or executing HMP info command got frozen indefinitely (subject to how large size the memory is being pinned). Is it possible to unblock those QMP request or HMP command from being executed (at least the read-only ones) while in migration? Yield from the load_setup corourtine and spawn another thread?Ok, I wasn't aware of that. I think we cannot yield in a coroutine and wait for an ioctl.I was wondering if we need a separate coroutine out of the general migration path to support this special code without overloading load_setup or its callers. For instance, unblock the source from sending guest rams while allow destination pin pages in parallel should be possible.Hi Si-Wei, I'm working on this, I think I'll be able to send a new version soon. Just a question, when the mapping is done in vhost_vdpa_dev_start as the current upstream master does, are you able to interact with QMP?
Hi Eugenio,Yes, the latest version works pretty well! Did not get to all of the QMP commands, but at least I can do read-only QMP without a problem. That is able to address our typical usages. Thanks for the prompt fix!
I've rebased my series on top the .load_setup series instead of the top 13 patches for 9.0, as there are some other dependent patches from this series to avoid duplicate work. Am debugging some problems I ran into after the code merge. Once they are sorted out I'll post my patch series soon!
Thanks, -Siwei
Thanks!Regardless, a separate thread is needed to carry out all the heavy lifting, i.e. ioctl(2) or write(2) syscalls to map&pin pages.One option that came to my mind is to effectively use another thread, and use a POSIX barrier (or equivalent on glib / QEMU) before finishing the migration.Yes, a separate thread is needed anyway.I'm not sure if there are more points where we can check the barrier and tell the migration to continue or stop though.I think there is, for e.g. what if the dma_map fails. There must be a check point for that.Another option is to effectively start doing these ioctls in an asynchronous way, io_uring cmds like, but I'd like to achieve this first.Yes, io_uring or any async API could be another option. Though this needs new uAPI through additional kernel facility to support. Anyway, it's up to you to decide. :) Regards, -SiweiHaving said, not sure if .load_setup is a good fit for what we want to do. Searching all current users of .load_setup, either the job can be done instantly or the task is time bound without trapping into kernel for too long. Maybe pinning is too special use case here... -SiweiFuture directions on top of this series may include: * Iterative migration of virtio-net devices, as it may reduce downtime per [1]. vhost-vdpa net can apply the configuration through CVQ in the destination while the source is still migrating. * Move more things ahead of migration time, like DRIVER_OK. * Check that the devices of the destination are valid, and cancel the migration in case it is not. [1] https://lore.kernel.org/qemu-devel/6c8ebb97-d546-3f1c-4cdd-54e23a566f61@nvidia.com/T/ Eugenio Pérez (18): vdpa: add VhostVDPAShared vdpa: move iova tree to the shared struct vdpa: move iova_range to vhost_vdpa_shared vdpa: move shadow_data to vhost_vdpa_shared vdpa: use vdpa shared for tracing vdpa: move file descriptor to vhost_vdpa_shared vdpa: move iotlb_batch_begin_sent to vhost_vdpa_shared vdpa: move backend_cap to vhost_vdpa_shared vdpa: remove msg type of vhost_vdpa vdpa: move iommu_list to vhost_vdpa_shared vdpa: use VhostVDPAShared in vdpa_dma_map and unmap vdpa: use dev_shared in vdpa_iommu vdpa: move memory listener to vhost_vdpa_shared vdpa: do not set virtio status bits if unneeded vdpa: add vhost_vdpa_load_setup vdpa: add vhost_vdpa_net_load_setup NetClient callback vdpa: use shadow_data instead of first device v->shadow_vqs_enabled virtio_net: register incremental migration handlers include/hw/virtio/vhost-vdpa.h | 43 +++++--- include/net/net.h | 4 + hw/net/virtio-net.c | 23 +++++ hw/virtio/vdpa-dev.c | 7 +- hw/virtio/vhost-vdpa.c | 183 ++++++++++++++++++--------------- net/vhost-vdpa.c | 127 ++++++++++++----------- hw/virtio/trace-events | 14 +-- 7 files changed, 239 insertions(+), 162 deletions(-)
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