Re: [Qemu-devel] [PATCH V5 16/18] virtio-pci: increase the maximum number of virtqueues to 513

[Jason Wang <jasowang@redhat.com>]

On 04/08/2015 12:54 AM, Alexander Graf wrote:
> On 04/01/2015 10:15 AM, 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.
>>
>> 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.
>>
>> 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 | 2 +-
>>   4 files changed, 16 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 9a5242a..02e3ce8 100644
>> --- a/hw/virtio/virtio-pci.c
>> +++ b/hw/virtio/virtio-pci.c
>> @@ -42,7 +42,7 @@
>>    * configuration space */
>>   #define VIRTIO_PCI_CONFIG_SIZE(dev)    
>> VIRTIO_PCI_CONFIG_OFF(msix_enabled(dev))
>>   -#define VIRTIO_PCI_QUEUE_MAX 64
>> +#define VIRTIO_PCI_QUEUE_MAX 513
>
> 513 is an interesting number. Any particular reason for it? Maybe this
> was mentioned before and I just missed it ;)
>
>
> Alex
>
>

The number were chose to match kernel limit for tuntap queues.  Recent
kernel allow up to 256 queues for tuntap, so 513 is in fact 256 * 2 plus
1 control vq.