Re: [PATCH] virtio: add the queue number check

[Paolo Bonzini]

On 23/12/19 15:25, Michael S. Tsirkin wrote:
> On Mon, Dec 23, 2019 at 12:02:18PM +0100, Paolo Bonzini wrote:
>> On 23/12/19 10:18, Yang Zhong wrote:
>>>   In this time, the queue number in the front-end block driver is 2, but
>>>   the queue number in qemu side is still 4. So the guest virtio_blk
>>>   driver will failed to create vq with backend.
>>
>> Where?
>>
>>>   There is no "set back"
>>>   mechnism for block driver to inform backend this new queue number.
>>>   So, i added this check in qemu side.
>>
>> Perhaps the guest kernel should still create the virtqueues, and just
>> not use them.  In any case, now that you have explained it, it is
>> certainly a guest bug.
> 
> Paolo do you understand where the bug is?

No, I asked above where does the virtio_blk driver fail to create the
virtqueues.  But it shouldn't; it is legal for the guest not to
configure all virtqueues, and QEMU knows to ignore the extra ones.  For
example, firmware can ignore virtio-scsi request queues above the first,
or ignore the virtio-scsi control and event queues (see
src/hw/virtio-scsi.c in SeaBIOS, it only calls vp_find_vq with index 2).

In particular this is the reason why request queues for virtio-scsi are
numbered 2 and above, rather than starting from zero: this way, the
guest can just pretend that unnecessary queues do not exist, and still
keep the virtqueue numbers consecutive.

> E.g. I see this in vhost user block:
> 
>     /* Kick right away to begin processing requests already in vring */
>     for (i = 0; i < s->dev.nvqs; i++) {
>         VirtQueue *kick_vq = virtio_get_queue(vdev, i);
> 
>         if (!virtio_queue_get_desc_addr(vdev, i)) {
>             continue;
>         }
>         event_notifier_set(virtio_queue_get_host_notifier(kick_vq));
>     }
> 
> which is an (admittedly hacky) want to skip VQs which
> were not configured by guest ....

Right, this is an example of QEMU ignoring extra virtqueues.

Paolo

> 
> 
>>>   Since the current virtio-blk and vhost-user-blk device always
>>>   defaultly use 1 queue, it's hard to find this issue.
>>>
>>>   I checked the guest kernel driver, virtio-scsi and virtio-blk all
>>>   have same check in their driver probe:
>>>
>>>   num_vqs = min_t(unsigned int, nr_cpu_ids, num_vqs);
>>>  
>>>   It's possible the guest driver has different queue number with qemu
>>>   side.
>>>
>>>   I also want to fix this issue from guest driver side, but currently there 
>>>   is no better solution to fix this issue.
>>>
>>>   By the way, i did not try scsi with this corner case, and only check
>>>   driver and qemu code to find same issue. thanks! 
>>>
>>>   Yang
>>>
>>>> Paolo
>>>>
>>>>> Signed-off-by: Yang Zhong <address@hidden>
>>>>> ---
>>>>>  hw/block/vhost-user-blk.c | 11 +++++++++++
>>>>>  hw/block/virtio-blk.c     | 11 ++++++++++-
>>>>>  hw/scsi/virtio-scsi.c     | 12 ++++++++++++
>>>>>  3 files changed, 33 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
>>>>> index 63da9bb619..250e72abe4 100644
>>>>> --- a/hw/block/vhost-user-blk.c
>>>>> +++ b/hw/block/vhost-user-blk.c
>>>>> @@ -23,6 +23,8 @@
>>>>>  #include "qom/object.h"
>>>>>  #include "hw/qdev-core.h"
>>>>>  #include "hw/qdev-properties.h"
>>>>> +#include "qemu/option.h"
>>>>> +#include "qemu/config-file.h"
>>>>>  #include "hw/virtio/vhost.h"
>>>>>  #include "hw/virtio/vhost-user-blk.h"
>>>>>  #include "hw/virtio/virtio.h"
>>>>> @@ -391,6 +393,7 @@ static void vhost_user_blk_device_realize(DeviceState 
>>>>> *dev, Error **errp)
>>>>>      VirtIODevice *vdev = VIRTIO_DEVICE(dev);
>>>>>      VHostUserBlk *s = VHOST_USER_BLK(vdev);
>>>>>      Error *err = NULL;
>>>>> +    unsigned cpus;
>>>>>      int i, ret;
>>>>>  
>>>>>      if (!s->chardev.chr) {
>>>>> @@ -403,6 +406,14 @@ static void 
>>>>> vhost_user_blk_device_realize(DeviceState *dev, Error **errp)
>>>>>          return;
>>>>>      }
>>>>>  
>>>>> +    cpus = 
>>>>> qemu_opt_get_number(qemu_opts_find(qemu_find_opts("smp-opts"), NULL),
>>>>> +                               "cpus", 0);
>>>>> +    if (s->num_queues > cpus ) {
>>>>> +        error_setg(errp, "vhost-user-blk: the queue number should be 
>>>>> equal "
>>>>> +                "or less than vcpu number");
>>>>> +        return;
>>>>> +    }
>>>>> +
>>>>>      if (!s->queue_size) {
>>>>>          error_setg(errp, "vhost-user-blk: queue size must be non-zero");
>>>>>          return;
>>>>> diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
>>>>> index d62e6377c2..b2f4d01148 100644
>>>>> --- a/hw/block/virtio-blk.c
>>>>> +++ b/hw/block/virtio-blk.c
>>>>> @@ -18,6 +18,8 @@
>>>>>  #include "qemu/error-report.h"
>>>>>  #include "qemu/main-loop.h"
>>>>>  #include "trace.h"
>>>>> +#include "qemu/option.h"
>>>>> +#include "qemu/config-file.h"
>>>>>  #include "hw/block/block.h"
>>>>>  #include "hw/qdev-properties.h"
>>>>>  #include "sysemu/blockdev.h"
>>>>> @@ -1119,7 +1121,7 @@ static void virtio_blk_device_realize(DeviceState 
>>>>> *dev, Error **errp)
>>>>>      VirtIOBlock *s = VIRTIO_BLK(dev);
>>>>>      VirtIOBlkConf *conf = &s->conf;
>>>>>      Error *err = NULL;
>>>>> -    unsigned i;
>>>>> +    unsigned i,cpus;
>>>>>  
>>>>>      if (!conf->conf.blk) {
>>>>>          error_setg(errp, "drive property not set");
>>>>> @@ -1133,6 +1135,13 @@ static void virtio_blk_device_realize(DeviceState 
>>>>> *dev, Error **errp)
>>>>>          error_setg(errp, "num-queues property must be larger than 0");
>>>>>          return;
>>>>>      }
>>>>> +    cpus = 
>>>>> qemu_opt_get_number(qemu_opts_find(qemu_find_opts("smp-opts"), NULL),
>>>>> +                               "cpus", 0);
>>>>> +    if (conf->num_queues > cpus ) {
>>>>> +        error_setg(errp, "virtio-blk: the queue number should be equal "
>>>>> +                "or less than vcpu number");
>>>>> +        return;
>>>>> +    }
>>>>>      if (!is_power_of_2(conf->queue_size) ||
>>>>>          conf->queue_size > VIRTQUEUE_MAX_SIZE) {
>>>>>          error_setg(errp, "invalid queue-size property (%" PRIu16 "), "
>>>>> diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
>>>>> index e8b2b64d09..8e3e44f6b9 100644
>>>>> --- a/hw/scsi/virtio-scsi.c
>>>>> +++ b/hw/scsi/virtio-scsi.c
>>>>> @@ -21,6 +21,8 @@
>>>>>  #include "qemu/error-report.h"
>>>>>  #include "qemu/iov.h"
>>>>>  #include "qemu/module.h"
>>>>> +#include "qemu/option.h"
>>>>> +#include "qemu/config-file.h"
>>>>>  #include "sysemu/block-backend.h"
>>>>>  #include "hw/qdev-properties.h"
>>>>>  #include "hw/scsi/scsi.h"
>>>>> @@ -880,6 +882,7 @@ void virtio_scsi_common_realize(DeviceState *dev,
>>>>>  {
>>>>>      VirtIODevice *vdev = VIRTIO_DEVICE(dev);
>>>>>      VirtIOSCSICommon *s = VIRTIO_SCSI_COMMON(dev);
>>>>> +    unsigned cpus;
>>>>>      int i;
>>>>>  
>>>>>      virtio_init(vdev, "virtio-scsi", VIRTIO_ID_SCSI,
>>>>> @@ -893,6 +896,15 @@ void virtio_scsi_common_realize(DeviceState *dev,
>>>>>          virtio_cleanup(vdev);
>>>>>          return;
>>>>>      }
>>>>> +
>>>>> +    cpus = 
>>>>> qemu_opt_get_number(qemu_opts_find(qemu_find_opts("smp-opts"), NULL),
>>>>> +                               "cpus", 0);
>>>>> +    if (s->conf.num_queues > cpus ) {
>>>>> +        error_setg(errp, "virtio-scsi: the queue number should be equal "
>>>>> +                "or less than vcpu number");
>>>>> +        return;
>>>>> +    }
>>>>> +
>>>>>      s->cmd_vqs = g_new0(VirtQueue *, s->conf.num_queues);
>>>>>      s->sense_size = VIRTIO_SCSI_SENSE_DEFAULT_SIZE;
>>>>>      s->cdb_size = VIRTIO_SCSI_CDB_DEFAULT_SIZE;
>>>>>
>>>
>