Re: [PATCH v1 1/3] virtio-pci: submit msi route changes in batch

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Feb 28, 2023 at 07:39:06PM +0800, Longpeng (Mike, Cloud Infrastructure Service Product Dept.) wrote:
> 
> 
> 在 2023/2/28 19:20, Longpeng (Mike, Cloud Infrastructure Service Product
> Dept.) 写道:
> > 
> > 
> > 在 2023/2/28 18:17, Michael S. Tsirkin 写道:
> > > On Tue, Feb 28, 2023 at 05:39:35PM +0800, Longpeng(Mike) wrote:
> > > > From: Longpeng <longpeng2@huawei.com>
> > > > 
> > > > The kvm_irqchip_commit_routes() is a time-intensive operation, it needs
> > > > scan and update all irqfds that are already assigned during each
> > > > invocation,
> > > > so more vectors means need more time to process them.
> > > 
> > > I think the real reason is it's the write side of RCU.
> > > 
> > 
> > Yes, so we can reduce the invocation of it in this way.
> > 
> > I'll send other optimizations in the next step, including irqbypass,
> > kvm_irqfd, etc.
> > 
> 
> Iterates the irqfds list is also time-consuming, it would iterate all
> existing irqfds when we commit, so the time complexity is O(n^2) without
> this patch.

Sounds good, pls include this in the commit log.

> > > > For virtio-pci, we
> > > > can just submit once when enabling vectors of a virtio-pci device.
> > > > 
> > > > This can reduce the downtime when migrating a VM with vhost-vdpa
> > > > devices.
> > > > 
> > > > Signed-off-by: Longpeng <longpeng2@huawei.com>
> > > > ---
> > > >   hw/virtio/virtio-pci.c | 24 +++++++++++++++++++++---
> > > >   1 file changed, 21 insertions(+), 3 deletions(-)
> > > > 
> > > > diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> > > > index 247325c193..22e76e3902 100644
> > > > --- a/hw/virtio/virtio-pci.c
> > > > +++ b/hw/virtio/virtio-pci.c
> > > > @@ -49,6 +49,19 @@
> > > >    * configuration space */
> > > >   #define VIRTIO_PCI_CONFIG_SIZE(dev)
> > > > VIRTIO_PCI_CONFIG_OFF(msix_enabled(dev))
> > > > +/* Protected by the BQL */
> > > > +static KVMRouteChange virtio_pci_route_change;
> > > > +
> > > > +static inline void virtio_pci_begin_route_changes(void)
> > > > +{
> > > > +    virtio_pci_route_change =
> > > > kvm_irqchip_begin_route_changes(kvm_state);
> > > > +}
> > > > +
> > > > +static inline void virtio_pci_commit_route_changes(void)
> > > > +{
> > > > +    kvm_irqchip_commit_route_changes(&virtio_pci_route_change);
> > > > +}
> > > > +
> > > >   static void virtio_pci_bus_new(VirtioBusState *bus, size_t bus_size,
> > > >                                  VirtIOPCIProxy *dev);
> > > >   static void virtio_pci_reset(DeviceState *qdev);
> > > > @@ -790,12 +803,11 @@ static int
> > > > kvm_virtio_pci_vq_vector_use(VirtIOPCIProxy *proxy,
> > > >       int ret;
> > > >       if (irqfd->users == 0) {
> > > > -        KVMRouteChange c = kvm_irqchip_begin_route_changes(kvm_state);
> > > > -        ret = kvm_irqchip_add_msi_route(&c, vector, &proxy->pci_dev);
> > > > +        ret =
> > > > kvm_irqchip_add_msi_route(&virtio_pci_route_change, vector,
> > > > +                                        &proxy->pci_dev);
> > > >           if (ret < 0) {
> > > >               return ret;
> > > >           }
> > > > -        kvm_irqchip_commit_route_changes(&c);
> > > >           irqfd->virq = ret;
> > > >       }
> > > >       irqfd->users++;
> > > > @@ -903,12 +915,18 @@ static int
> > > > kvm_virtio_pci_vector_vq_use(VirtIOPCIProxy *proxy, int nvqs)
> > > >       int ret = 0;
> > > >       VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
> > > > +    virtio_pci_begin_route_changes();
> > > > +
> > > >       for (queue_no = 0; queue_no < nvqs; queue_no++) {
> > > >           if (!virtio_queue_get_num(vdev, queue_no)) {
> > > > +            virtio_pci_commit_route_changes();
> > > >               return -1;
> > > >           }
> > > >           ret = kvm_virtio_pci_vector_use_one(proxy, queue_no);
> > > >       }
> > > > +
> > > > +    virtio_pci_commit_route_changes();
> > > > +
> > > >       return ret;
> > > >   }
> > > > -- 
> > > > 2.23.0
> > > 
> > > .