Re: [Qemu-devel] [PATCH V7 15/16] virtio-pci: increase the maximum number of virtqueues to 513

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

On Tue, Apr 28, 2015 at 11:12:10AM +0800, Jason Wang wrote:
> 
> 
> On Mon, Apr 27, 2015 at 7:02 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> >On Thu, Apr 23, 2015 at 02:21:48PM +0800, Jason Wang wrote:
> >> This patch increases the maximum number of virtqueues for pci from 64
> >> to 513. This will allow booting a virtio-net-pci device with 256 queue
> >> pairs on recent Linux host (which supports up to 256 tuntap queue
> >>pairs).
> >> To keep migration compatibility, 64 was kept for legacy machine
> >> types. This is because qemu in fact allows guest to probe the limit of
> >> virtqueues through trying to set queue_sel. So for legacy machine
> >> type, we should make sure setting queue_sel to more than 63 won't
> >> make effect.
> >
> >This isn't a documented interface, and no guest that I know of does
> >this.  Accordingly, I think we should drop everything except the
> >hw/virtio/virtio-pci.c change.
> 
> We leave a chance for guest to use such undocumented behavior, so
> technically we'd better keep it and it maybe too late for us to fix if we
> find such a guest in the future. And consider keeping this compatibility was
> really not hard, so I suggest to include this.

Reminds me of  https://xkcd.com/1172/
We don't do this kind of thing.

> >
> >
> >> Cc: Paolo Bonzini <pbonzini@redhat.com>
> >> Cc: Richard Henderson <rth@twiddle.net>
> >> Cc: Michael S. Tsirkin <mst@redhat.com>
> >> Cc: Alexander Graf <agraf@suse.de>
> >> Cc: qemu-ppc@nongnu.org
> >> Signed-off-by: Jason Wang <jasowang@redhat.com>
> >> ---
> >>  hw/i386/pc_piix.c      | 5 +++++
> >>  hw/i386/pc_q35.c       | 5 +++++
> >>  hw/ppc/spapr.c         | 5 +++++
> >>  hw/virtio/virtio-pci.c | 6 +++++-
> >>  4 files changed, 20 insertions(+), 1 deletion(-)
> >> diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
> >> index 212e263..6e098ce 100644
> >> --- a/hw/i386/pc_piix.c
> >> +++ b/hw/i386/pc_piix.c
> >> @@ -49,6 +49,7 @@
> >>  #include "hw/acpi/acpi.h"
> >>  #include "cpu.h"
> >>  #include "qemu/error-report.h"
> >> +#include "hw/virtio/virtio-bus.h"
> >>  #ifdef CONFIG_XEN
> >>  #  include <xen/hvm/hvm_info_table.h>
> >>  #endif
> >> @@ -312,6 +313,10 @@ static void pc_init_pci(MachineState *machine)
> >>  static void pc_compat_2_3(MachineState *machine)
> >>  {
> >> +    ObjectClass *klass = object_class_by_name("virtio-pci-bus");
> >> +    VirtioBusClass *k = VIRTIO_BUS_CLASS(klass);
> >> +
> >> +    k->queue_max = 64;
> >>  }
> >>  static void pc_compat_2_2(MachineState *machine)
> >> diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
> >> index e67f2de..ff7c414 100644
> >> --- a/hw/i386/pc_q35.c
> >> +++ b/hw/i386/pc_q35.c
> >> @@ -45,6 +45,7 @@
> >>  #include "hw/usb.h"
> >>  #include "hw/cpu/icc_bus.h"
> >>  #include "qemu/error-report.h"
> >> +#include "include/hw/virtio/virtio-bus.h"
> >>  /* ICH9 AHCI has 6 ports */
> >>  #define MAX_SATA_PORTS     6
> >> @@ -291,6 +292,10 @@ static void pc_q35_init(MachineState *machine)
> >>  static void pc_compat_2_3(MachineState *machine)
> >>  {
> >> +    ObjectClass *klass = object_class_by_name("virtio-pci-bus");
> >> +    VirtioBusClass *k = VIRTIO_BUS_CLASS(klass);
> >> +
> >> +    k->queue_max = 64;
> >>  }
> >>  static void pc_compat_2_2(MachineState *machine)
> >> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> >> index 8e43aa2..ee8f6a3 100644
> >> --- a/hw/ppc/spapr.c
> >> +++ b/hw/ppc/spapr.c
> >> @@ -50,6 +50,7 @@
> >>  #include "hw/pci/pci.h"
> >>  #include "hw/scsi/scsi.h"
> >>  #include "hw/virtio/virtio-scsi.h"
> >> +#include "hw/virtio/virtio-bus.h"
> >>  #include "exec/address-spaces.h"
> >>  #include "hw/usb.h"
> >> @@ -1827,6 +1828,10 @@ static const TypeInfo spapr_machine_info = {
> >>  static void spapr_compat_2_3(Object *obj)
> >>  {
> >> +    ObjectClass *klass = object_class_by_name("virtio-pci-bus");
> >> +    VirtioBusClass *k = VIRTIO_BUS_CLASS(klass);
> >> +
> >> +    k->queue_max = 64;
> >>  }
> >>  static void spapr_compat_2_2(Object *obj)
> >> diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> >> index 7d01500..c510cb7 100644
> >> --- a/hw/virtio/virtio-pci.c
> >> +++ b/hw/virtio/virtio-pci.c
> >> @@ -42,7 +42,11 @@
> >>   * configuration space */
> >>  #define VIRTIO_PCI_CONFIG_SIZE(dev)
> >>VIRTIO_PCI_CONFIG_OFF(msix_enabled(dev))
> >> -#define VIRTIO_PCI_QUEUE_MAX 64
> >> +/* The number was chose to be greater than both the the number of max
> >> + * vcpus supported by host and the number of max tuntap queues support
> >> + * by host and also leave some spaces for future.
> >> + */
> >> +#define VIRTIO_PCI_QUEUE_MAX 1024
> >>     static void virtio_pci_bus_new(VirtioBusState *bus, size_t
> >>bus_size,
> >>                                 VirtIOPCIProxy *dev);
> >> --  2.1.0