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

[Longpeng (Mike, Cloud Infrastructure Service Product Dept.)]

在 2023/2/28 18:18, 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. 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.
>
> can in what sense?  does it or does it not? by how much?

I've replied in patch 3.

> > 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
>
> .