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[PATCH v4 3/5] virtio-scsi: default num_queues to -smp N

Stefan Hajnoczi posted 5 patches 5 years, 5 months ago
There is a newer version of this series
[PATCH v4 3/5] virtio-scsi: default num_queues to -smp N
Posted by Stefan Hajnoczi 5 years, 5 months ago 
Automatically size the number of virtio-scsi-pci, vhost-scsi-pci, and
vhost-user-scsi-pci request virtqueues to match the number of vCPUs.
Other transports continue to default to 1 request virtqueue.

A 1:1 virtqueue:vCPU mapping ensures that completion interrupts are
handled on the same vCPU that submitted the request.  No IPI is
necessary to complete an I/O request and performance is improved.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 include/hw/virtio/virtio-scsi.h |  2 ++
 hw/core/machine.c               |  6 +++++-
 hw/scsi/vhost-scsi.c            |  3 ++-
 hw/scsi/vhost-user-scsi.c       |  3 ++-
 hw/scsi/virtio-scsi.c           |  6 +++++-
 hw/virtio/vhost-scsi-pci.c      | 10 +++++++---
 hw/virtio/vhost-user-scsi-pci.c | 10 +++++++---
 hw/virtio/virtio-scsi-pci.c     | 10 +++++++---
 8 files changed, 37 insertions(+), 13 deletions(-)

diff --git a/include/hw/virtio/virtio-scsi.h b/include/hw/virtio/virtio-scsi.h
index 9f293bcb80..c0b8e4dd7e 100644
--- a/include/hw/virtio/virtio-scsi.h
+++ b/include/hw/virtio/virtio-scsi.h
@@ -39,6 +39,8 @@
 /* Number of virtqueues that are always present */
 #define VIRTIO_SCSI_VQ_NUM_FIXED    2
 
+#define VIRTIO_SCSI_AUTO_NUM_QUEUES UINT32_MAX
+
 typedef struct virtio_scsi_cmd_req VirtIOSCSICmdReq;
 typedef struct virtio_scsi_cmd_resp VirtIOSCSICmdResp;
 typedef struct virtio_scsi_ctrl_tmf_req VirtIOSCSICtrlTMFReq;
diff --git a/hw/core/machine.c b/hw/core/machine.c
index bb3a7b18b1..df7664bc8d 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -28,7 +28,11 @@
 #include "hw/mem/nvdimm.h"
 #include "migration/vmstate.h"
 
-GlobalProperty hw_compat_5_0[] = {};
+GlobalProperty hw_compat_5_0[] = {
+    { "virtio-scsi-device", "num_queues", "1"},
+    { "vhost-scsi", "num_queues", "1"},
+    { "vhost-user-scsi", "num_queues", "1"},
+};
 const size_t hw_compat_5_0_len = G_N_ELEMENTS(hw_compat_5_0);
 
 GlobalProperty hw_compat_4_2[] = {
diff --git a/hw/scsi/vhost-scsi.c b/hw/scsi/vhost-scsi.c
index c1b012aea4..5e3bc319ab 100644
--- a/hw/scsi/vhost-scsi.c
+++ b/hw/scsi/vhost-scsi.c
@@ -272,7 +272,8 @@ static Property vhost_scsi_properties[] = {
     DEFINE_PROP_STRING("vhostfd", VirtIOSCSICommon, conf.vhostfd),
     DEFINE_PROP_STRING("wwpn", VirtIOSCSICommon, conf.wwpn),
     DEFINE_PROP_UINT32("boot_tpgt", VirtIOSCSICommon, conf.boot_tpgt, 0),
-    DEFINE_PROP_UINT32("num_queues", VirtIOSCSICommon, conf.num_queues, 1),
+    DEFINE_PROP_UINT32("num_queues", VirtIOSCSICommon, conf.num_queues,
+                       VIRTIO_SCSI_AUTO_NUM_QUEUES),
     DEFINE_PROP_UINT32("virtqueue_size", VirtIOSCSICommon, conf.virtqueue_size,
                        128),
     DEFINE_PROP_BOOL("seg_max_adjust", VirtIOSCSICommon, conf.seg_max_adjust,
diff --git a/hw/scsi/vhost-user-scsi.c b/hw/scsi/vhost-user-scsi.c
index f8bd158c31..4b56de97a8 100644
--- a/hw/scsi/vhost-user-scsi.c
+++ b/hw/scsi/vhost-user-scsi.c
@@ -163,7 +163,8 @@ static void vhost_user_scsi_unrealize(DeviceState *dev)
 static Property vhost_user_scsi_properties[] = {
     DEFINE_PROP_CHR("chardev", VirtIOSCSICommon, conf.chardev),
     DEFINE_PROP_UINT32("boot_tpgt", VirtIOSCSICommon, conf.boot_tpgt, 0),
-    DEFINE_PROP_UINT32("num_queues", VirtIOSCSICommon, conf.num_queues, 1),
+    DEFINE_PROP_UINT32("num_queues", VirtIOSCSICommon, conf.num_queues,
+                       VIRTIO_SCSI_AUTO_NUM_QUEUES),
     DEFINE_PROP_UINT32("virtqueue_size", VirtIOSCSICommon, conf.virtqueue_size,
                        128),
     DEFINE_PROP_UINT32("max_sectors", VirtIOSCSICommon, conf.max_sectors,
diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
index f3d60683bd..f055ae7389 100644
--- a/hw/scsi/virtio-scsi.c
+++ b/hw/scsi/virtio-scsi.c
@@ -891,6 +891,9 @@ void virtio_scsi_common_realize(DeviceState *dev,
     virtio_init(vdev, "virtio-scsi", VIRTIO_ID_SCSI,
                 sizeof(VirtIOSCSIConfig));
 
+    if (s->conf.num_queues == VIRTIO_SCSI_AUTO_NUM_QUEUES) {
+        s->conf.num_queues = 1;
+    }
     if (s->conf.num_queues == 0 ||
             s->conf.num_queues > VIRTIO_QUEUE_MAX - VIRTIO_SCSI_VQ_NUM_FIXED) {
         error_setg(errp, "Invalid number of queues (= %" PRIu32 "), "
@@ -964,7 +967,8 @@ static void virtio_scsi_device_unrealize(DeviceState *dev)
 }
 
 static Property virtio_scsi_properties[] = {
-    DEFINE_PROP_UINT32("num_queues", VirtIOSCSI, parent_obj.conf.num_queues, 1),
+    DEFINE_PROP_UINT32("num_queues", VirtIOSCSI, parent_obj.conf.num_queues,
+                       VIRTIO_SCSI_AUTO_NUM_QUEUES),
     DEFINE_PROP_UINT32("virtqueue_size", VirtIOSCSI,
                                          parent_obj.conf.virtqueue_size, 256),
     DEFINE_PROP_BOOL("seg_max_adjust", VirtIOSCSI,
diff --git a/hw/virtio/vhost-scsi-pci.c b/hw/virtio/vhost-scsi-pci.c
index 2b25db9a3c..9f377a84cf 100644
--- a/hw/virtio/vhost-scsi-pci.c
+++ b/hw/virtio/vhost-scsi-pci.c
@@ -47,11 +47,15 @@ static void vhost_scsi_pci_realize(VirtIOPCIProxy *vpci_dev, Error **errp)
 {
     VHostSCSIPCI *dev = VHOST_SCSI_PCI(vpci_dev);
     DeviceState *vdev = DEVICE(&dev->vdev);
-    VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(vdev);
+    VirtIOSCSIConf *conf = &dev->vdev.parent_obj.parent_obj.conf;
+
+    if (conf->num_queues == VIRTIO_SCSI_AUTO_NUM_QUEUES) {
+        conf->num_queues =
+            virtio_pci_optimal_num_queues(VIRTIO_SCSI_VQ_NUM_FIXED);
+    }
 
     if (vpci_dev->nvectors == DEV_NVECTORS_UNSPECIFIED) {
-        vpci_dev->nvectors = vs->conf.num_queues +
-                             VIRTIO_SCSI_VQ_NUM_FIXED + 1;
+        vpci_dev->nvectors = conf->num_queues + VIRTIO_SCSI_VQ_NUM_FIXED + 1;
     }
 
     qdev_set_parent_bus(vdev, BUS(&vpci_dev->bus));
diff --git a/hw/virtio/vhost-user-scsi-pci.c b/hw/virtio/vhost-user-scsi-pci.c
index 80710ab170..2a4c0d27f1 100644
--- a/hw/virtio/vhost-user-scsi-pci.c
+++ b/hw/virtio/vhost-user-scsi-pci.c
@@ -53,11 +53,15 @@ static void vhost_user_scsi_pci_realize(VirtIOPCIProxy *vpci_dev, Error **errp)
 {
     VHostUserSCSIPCI *dev = VHOST_USER_SCSI_PCI(vpci_dev);
     DeviceState *vdev = DEVICE(&dev->vdev);
-    VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(vdev);
+    VirtIOSCSIConf *conf = &dev->vdev.parent_obj.parent_obj.conf;
+
+    if (conf->num_queues == VIRTIO_SCSI_AUTO_NUM_QUEUES) {
+        conf->num_queues =
+            virtio_pci_optimal_num_queues(VIRTIO_SCSI_VQ_NUM_FIXED);
+    }
 
     if (vpci_dev->nvectors == DEV_NVECTORS_UNSPECIFIED) {
-        vpci_dev->nvectors = vs->conf.num_queues +
-                             VIRTIO_SCSI_VQ_NUM_FIXED + 1;
+        vpci_dev->nvectors = conf->num_queues + VIRTIO_SCSI_VQ_NUM_FIXED + 1;
     }
 
     qdev_set_parent_bus(vdev, BUS(&vpci_dev->bus));
diff --git a/hw/virtio/virtio-scsi-pci.c b/hw/virtio/virtio-scsi-pci.c
index c52d68053a..c3e2f8625a 100644
--- a/hw/virtio/virtio-scsi-pci.c
+++ b/hw/virtio/virtio-scsi-pci.c
@@ -46,13 +46,17 @@ static void virtio_scsi_pci_realize(VirtIOPCIProxy *vpci_dev, Error **errp)
 {
     VirtIOSCSIPCI *dev = VIRTIO_SCSI_PCI(vpci_dev);
     DeviceState *vdev = DEVICE(&dev->vdev);
-    VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(vdev);
     DeviceState *proxy = DEVICE(vpci_dev);
+    VirtIOSCSIConf *conf = &dev->vdev.parent_obj.conf;
     char *bus_name;
 
+    if (conf->num_queues == VIRTIO_SCSI_AUTO_NUM_QUEUES) {
+        conf->num_queues =
+            virtio_pci_optimal_num_queues(VIRTIO_SCSI_VQ_NUM_FIXED);
+    }
+
     if (vpci_dev->nvectors == DEV_NVECTORS_UNSPECIFIED) {
-        vpci_dev->nvectors = vs->conf.num_queues +
-                             VIRTIO_SCSI_VQ_NUM_FIXED + 1;
+        vpci_dev->nvectors = conf->num_queues + VIRTIO_SCSI_VQ_NUM_FIXED + 1;
     }
 
     /*
-- 
2.25.4
Re: [PATCH v4 3/5] virtio-scsi: default num_queues to -smp N
Posted by Daniel P. Berrangé 5 years, 5 months ago 
On Wed, May 27, 2020 at 11:29:23AM +0100, Stefan Hajnoczi wrote:
> Automatically size the number of virtio-scsi-pci, vhost-scsi-pci, and
> vhost-user-scsi-pci request virtqueues to match the number of vCPUs.
> Other transports continue to default to 1 request virtqueue.

IIRC this was raised on earlier versions of the series, but i don't
recall the outcome and no caveats are mentioned here...

Is this default still valid for very large $vCPUs. eg if I run QEMU
with "-smp 512" or even larger (I've seen people discussing 1000's of
CPUs), is this going to cause problems with the virtio-scsi default
queue counts ? Is there such a thing as "too large" for the num
of queues setting ?   num vectors defaults to a value derived from
num queues, so is there concept of "too large" for num of vectors
setting ?

Ideally the commit message would answer these questions for future
reference.  Same for the next patch to virtio-blk

> 
> A 1:1 virtqueue:vCPU mapping ensures that completion interrupts are
> handled on the same vCPU that submitted the request.  No IPI is
> necessary to complete an I/O request and performance is improved.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  include/hw/virtio/virtio-scsi.h |  2 ++
>  hw/core/machine.c               |  6 +++++-
>  hw/scsi/vhost-scsi.c            |  3 ++-
>  hw/scsi/vhost-user-scsi.c       |  3 ++-
>  hw/scsi/virtio-scsi.c           |  6 +++++-
>  hw/virtio/vhost-scsi-pci.c      | 10 +++++++---
>  hw/virtio/vhost-user-scsi-pci.c | 10 +++++++---
>  hw/virtio/virtio-scsi-pci.c     | 10 +++++++---
>  8 files changed, 37 insertions(+), 13 deletions(-)
> 
> diff --git a/include/hw/virtio/virtio-scsi.h b/include/hw/virtio/virtio-scsi.h
> index 9f293bcb80..c0b8e4dd7e 100644
> --- a/include/hw/virtio/virtio-scsi.h
> +++ b/include/hw/virtio/virtio-scsi.h
> @@ -39,6 +39,8 @@
>  /* Number of virtqueues that are always present */
>  #define VIRTIO_SCSI_VQ_NUM_FIXED    2
>  
> +#define VIRTIO_SCSI_AUTO_NUM_QUEUES UINT32_MAX
> +
>  typedef struct virtio_scsi_cmd_req VirtIOSCSICmdReq;
>  typedef struct virtio_scsi_cmd_resp VirtIOSCSICmdResp;
>  typedef struct virtio_scsi_ctrl_tmf_req VirtIOSCSICtrlTMFReq;
> diff --git a/hw/core/machine.c b/hw/core/machine.c
> index bb3a7b18b1..df7664bc8d 100644
> --- a/hw/core/machine.c
> +++ b/hw/core/machine.c
> @@ -28,7 +28,11 @@
>  #include "hw/mem/nvdimm.h"
>  #include "migration/vmstate.h"
>  
> -GlobalProperty hw_compat_5_0[] = {};
> +GlobalProperty hw_compat_5_0[] = {
> +    { "virtio-scsi-device", "num_queues", "1"},
> +    { "vhost-scsi", "num_queues", "1"},
> +    { "vhost-user-scsi", "num_queues", "1"},
> +};
>  const size_t hw_compat_5_0_len = G_N_ELEMENTS(hw_compat_5_0);
>  
>  GlobalProperty hw_compat_4_2[] = {
> diff --git a/hw/scsi/vhost-scsi.c b/hw/scsi/vhost-scsi.c
> index c1b012aea4..5e3bc319ab 100644
> --- a/hw/scsi/vhost-scsi.c
> +++ b/hw/scsi/vhost-scsi.c
> @@ -272,7 +272,8 @@ static Property vhost_scsi_properties[] = {
>      DEFINE_PROP_STRING("vhostfd", VirtIOSCSICommon, conf.vhostfd),
>      DEFINE_PROP_STRING("wwpn", VirtIOSCSICommon, conf.wwpn),
>      DEFINE_PROP_UINT32("boot_tpgt", VirtIOSCSICommon, conf.boot_tpgt, 0),
> -    DEFINE_PROP_UINT32("num_queues", VirtIOSCSICommon, conf.num_queues, 1),
> +    DEFINE_PROP_UINT32("num_queues", VirtIOSCSICommon, conf.num_queues,
> +                       VIRTIO_SCSI_AUTO_NUM_QUEUES),
>      DEFINE_PROP_UINT32("virtqueue_size", VirtIOSCSICommon, conf.virtqueue_size,
>                         128),
>      DEFINE_PROP_BOOL("seg_max_adjust", VirtIOSCSICommon, conf.seg_max_adjust,
> diff --git a/hw/scsi/vhost-user-scsi.c b/hw/scsi/vhost-user-scsi.c
> index f8bd158c31..4b56de97a8 100644
> --- a/hw/scsi/vhost-user-scsi.c
> +++ b/hw/scsi/vhost-user-scsi.c
> @@ -163,7 +163,8 @@ static void vhost_user_scsi_unrealize(DeviceState *dev)
>  static Property vhost_user_scsi_properties[] = {
>      DEFINE_PROP_CHR("chardev", VirtIOSCSICommon, conf.chardev),
>      DEFINE_PROP_UINT32("boot_tpgt", VirtIOSCSICommon, conf.boot_tpgt, 0),
> -    DEFINE_PROP_UINT32("num_queues", VirtIOSCSICommon, conf.num_queues, 1),
> +    DEFINE_PROP_UINT32("num_queues", VirtIOSCSICommon, conf.num_queues,
> +                       VIRTIO_SCSI_AUTO_NUM_QUEUES),
>      DEFINE_PROP_UINT32("virtqueue_size", VirtIOSCSICommon, conf.virtqueue_size,
>                         128),
>      DEFINE_PROP_UINT32("max_sectors", VirtIOSCSICommon, conf.max_sectors,
> diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
> index f3d60683bd..f055ae7389 100644
> --- a/hw/scsi/virtio-scsi.c
> +++ b/hw/scsi/virtio-scsi.c
> @@ -891,6 +891,9 @@ void virtio_scsi_common_realize(DeviceState *dev,
>      virtio_init(vdev, "virtio-scsi", VIRTIO_ID_SCSI,
>                  sizeof(VirtIOSCSIConfig));
>  
> +    if (s->conf.num_queues == VIRTIO_SCSI_AUTO_NUM_QUEUES) {
> +        s->conf.num_queues = 1;
> +    }
>      if (s->conf.num_queues == 0 ||
>              s->conf.num_queues > VIRTIO_QUEUE_MAX - VIRTIO_SCSI_VQ_NUM_FIXED) {
>          error_setg(errp, "Invalid number of queues (= %" PRIu32 "), "
> @@ -964,7 +967,8 @@ static void virtio_scsi_device_unrealize(DeviceState *dev)
>  }
>  
>  static Property virtio_scsi_properties[] = {
> -    DEFINE_PROP_UINT32("num_queues", VirtIOSCSI, parent_obj.conf.num_queues, 1),
> +    DEFINE_PROP_UINT32("num_queues", VirtIOSCSI, parent_obj.conf.num_queues,
> +                       VIRTIO_SCSI_AUTO_NUM_QUEUES),
>      DEFINE_PROP_UINT32("virtqueue_size", VirtIOSCSI,
>                                           parent_obj.conf.virtqueue_size, 256),
>      DEFINE_PROP_BOOL("seg_max_adjust", VirtIOSCSI,
> diff --git a/hw/virtio/vhost-scsi-pci.c b/hw/virtio/vhost-scsi-pci.c
> index 2b25db9a3c..9f377a84cf 100644
> --- a/hw/virtio/vhost-scsi-pci.c
> +++ b/hw/virtio/vhost-scsi-pci.c
> @@ -47,11 +47,15 @@ static void vhost_scsi_pci_realize(VirtIOPCIProxy *vpci_dev, Error **errp)
>  {
>      VHostSCSIPCI *dev = VHOST_SCSI_PCI(vpci_dev);
>      DeviceState *vdev = DEVICE(&dev->vdev);
> -    VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(vdev);
> +    VirtIOSCSIConf *conf = &dev->vdev.parent_obj.parent_obj.conf;
> +
> +    if (conf->num_queues == VIRTIO_SCSI_AUTO_NUM_QUEUES) {
> +        conf->num_queues =
> +            virtio_pci_optimal_num_queues(VIRTIO_SCSI_VQ_NUM_FIXED);
> +    }
>  
>      if (vpci_dev->nvectors == DEV_NVECTORS_UNSPECIFIED) {
> -        vpci_dev->nvectors = vs->conf.num_queues +
> -                             VIRTIO_SCSI_VQ_NUM_FIXED + 1;
> +        vpci_dev->nvectors = conf->num_queues + VIRTIO_SCSI_VQ_NUM_FIXED + 1;
>      }
>  
>      qdev_set_parent_bus(vdev, BUS(&vpci_dev->bus));
> diff --git a/hw/virtio/vhost-user-scsi-pci.c b/hw/virtio/vhost-user-scsi-pci.c
> index 80710ab170..2a4c0d27f1 100644
> --- a/hw/virtio/vhost-user-scsi-pci.c
> +++ b/hw/virtio/vhost-user-scsi-pci.c
> @@ -53,11 +53,15 @@ static void vhost_user_scsi_pci_realize(VirtIOPCIProxy *vpci_dev, Error **errp)
>  {
>      VHostUserSCSIPCI *dev = VHOST_USER_SCSI_PCI(vpci_dev);
>      DeviceState *vdev = DEVICE(&dev->vdev);
> -    VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(vdev);
> +    VirtIOSCSIConf *conf = &dev->vdev.parent_obj.parent_obj.conf;
> +
> +    if (conf->num_queues == VIRTIO_SCSI_AUTO_NUM_QUEUES) {
> +        conf->num_queues =
> +            virtio_pci_optimal_num_queues(VIRTIO_SCSI_VQ_NUM_FIXED);
> +    }
>  
>      if (vpci_dev->nvectors == DEV_NVECTORS_UNSPECIFIED) {
> -        vpci_dev->nvectors = vs->conf.num_queues +
> -                             VIRTIO_SCSI_VQ_NUM_FIXED + 1;
> +        vpci_dev->nvectors = conf->num_queues + VIRTIO_SCSI_VQ_NUM_FIXED + 1;
>      }
>  
>      qdev_set_parent_bus(vdev, BUS(&vpci_dev->bus));
> diff --git a/hw/virtio/virtio-scsi-pci.c b/hw/virtio/virtio-scsi-pci.c
> index c52d68053a..c3e2f8625a 100644
> --- a/hw/virtio/virtio-scsi-pci.c
> +++ b/hw/virtio/virtio-scsi-pci.c
> @@ -46,13 +46,17 @@ static void virtio_scsi_pci_realize(VirtIOPCIProxy *vpci_dev, Error **errp)
>  {
>      VirtIOSCSIPCI *dev = VIRTIO_SCSI_PCI(vpci_dev);
>      DeviceState *vdev = DEVICE(&dev->vdev);
> -    VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(vdev);
>      DeviceState *proxy = DEVICE(vpci_dev);
> +    VirtIOSCSIConf *conf = &dev->vdev.parent_obj.conf;
>      char *bus_name;
>  
> +    if (conf->num_queues == VIRTIO_SCSI_AUTO_NUM_QUEUES) {
> +        conf->num_queues =
> +            virtio_pci_optimal_num_queues(VIRTIO_SCSI_VQ_NUM_FIXED);
> +    }
> +
>      if (vpci_dev->nvectors == DEV_NVECTORS_UNSPECIFIED) {
> -        vpci_dev->nvectors = vs->conf.num_queues +
> -                             VIRTIO_SCSI_VQ_NUM_FIXED + 1;
> +        vpci_dev->nvectors = conf->num_queues + VIRTIO_SCSI_VQ_NUM_FIXED + 1;
>      }
>  
>      /*
> -- 
> 2.25.4
> 

Regards,
Daniel
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Re: [PATCH v4 3/5] virtio-scsi: default num_queues to -smp N
Posted by Stefan Hajnoczi 5 years, 5 months ago 
On Wed, May 27, 2020 at 11:38:17AM +0100, Daniel P. Berrangé wrote:
> On Wed, May 27, 2020 at 11:29:23AM +0100, Stefan Hajnoczi wrote:
> > Automatically size the number of virtio-scsi-pci, vhost-scsi-pci, and
> > vhost-user-scsi-pci request virtqueues to match the number of vCPUs.
> > Other transports continue to default to 1 request virtqueue.
> 
> IIRC this was raised on earlier versions of the series, but i don't
> recall the outcome and no caveats are mentioned here...
> 
> Is this default still valid for very large $vCPUs. eg if I run QEMU
> with "-smp 512" or even larger (I've seen people discussing 1000's of
> CPUs), is this going to cause problems with the virtio-scsi default
> queue counts ? Is there such a thing as "too large" for the num
> of queues setting ?   num vectors defaults to a value derived from
> num queues, so is there concept of "too large" for num of vectors
> setting ?
> 
> Ideally the commit message would answer these questions for future
> reference.  Same for the next patch to virtio-blk

Good point. Actually this patch and the virtio-blk ones no longer
contain the queue number policy. The new virtio_pci_optimal_num_queues()
function encapsulates the policy to avoid duplication. I'll resend and
update that patch with the full rationale.

Thanks!

Patchew 2.1-devel-b67e4c2

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