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

[Jason Wang <jasowang@redhat.com>]

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.
  
> 
> 
>>  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