Il 31/01/2013 12:25, Michael S. Tsirkin ha scritto: > On Thu, Jan 31, 2013 at 10:13:11AM +0200, Gal Hammer wrote: >> Hi, >> >> How many msi-x vectors should be allocated for the virtio-serial device? >> >> I'm asking this as it seems that a proposed patch >> (http://lists.gnu.org/archive/html/qemu-devel/2011-12/msg02094.html) >> was not accepted and I re-encountered this issue while trying to >> upgrade the virtio-serial's Windows driver to use msi-x vectors. >> >> The virtio-serial's Linux module tries to use one vector per >> virtqueue (every serial port have two virtqueues) with a fall back >> to using only two vectors >> (http://lxr.linux.no/linux+v3.7.2/drivers/virtio/virtio_pci.c#L539). >> The problem is that qemu's virtio-pci device allocate less vectors >> than the modules expects. So, for example, if a serial device have >> 16 ports, 17 vectors are allocated. The module tries to use 34 >> vectors, fails and choose to use only 2, leaving 15 unused vectors. >> >> Is it possible to increase the vectors number from >> "proxy->serial.max_virtserial_ports + 1" to >> "(proxy->serial.max_virtserial_ports + 1) * 2"? >> >> Thanks, >> >> Gal. > > Allocated MSI-X vectors on x86 are a limited resource. Let's not waste > them. From what you say virtio serial works fine with two > vectors and I do not see any reason to let us use more > until someone can show an important workload where this helps. > > So I think we need something like the below, but we also > need to handle 1.3 and older compatibility so the > patch below shouldn't be applied. Want to try > writing up the complete patch?
I think the patch should instead be something like diff --git a/hw/virtio-pci.c b/hw/virtio-pci.c index 9abbcdf..24e8232 100644 --- a/hw/virtio-pci.c +++ b/hw/virtio-pci.c @@ -1154,7 +1154,7 @@ static const TypeInfo virtio_net_info = { static Property virtio_serial_properties[] = { DEFINE_PROP_BIT("ioeventfd", VirtIOPCIProxy, flags, VIRTIO_PCI_FLAG_USE_IOEVENTFD_BIT, true), - DEFINE_PROP_UINT32("vectors", VirtIOPCIProxy, nvectors, DEV_NVECTORS_UNSPECIFIED), + DEFINE_PROP_UINT32("vectors", VirtIOPCIProxy, nvectors, 2), DEFINE_PROP_HEX32("class", VirtIOPCIProxy, class_code, 0), DEFINE_VIRTIO_COMMON_FEATURES(VirtIOPCIProxy, host_features), DEFINE_PROP_UINT32("max_ports", VirtIOPCIProxy, serial.max_virtserial_ports, 31), plus the backwards-compatibility stuff. Paolo > --> > > serial: use 2 vectors by default > > Guests only utilize 2 vectors and this seems to be enough, > so let's only allocate as much. > serial is likely not so performance intensive to require more, > but if yes users can always override the nvectors property. > > Signed-off-by: Michael S. Tsirkin <m...@redhat.com> > > --- > > diff --git a/hw/virtio-pci.c b/hw/virtio-pci.c > index 9abbcdf..bb0f60e 100644 > --- a/hw/virtio-pci.c > +++ b/hw/virtio-pci.c > @@ -975,8 +975,7 @@ static int virtio_serial_init_pci(PCIDevice *pci_dev) > if (!vdev) { > return -1; > } > - vdev->nvectors = proxy->nvectors == DEV_NVECTORS_UNSPECIFIED > - ? proxy->serial.max_virtserial_ports > + 1 > + vdev->nvectors = proxy->nvectors == DEV_NVECTORS_UNSPECIFIED ? 2 > : proxy->nvectors; > virtio_init_pci(proxy, vdev); > proxy->nvectors = vdev->nvectors; >