Re: [PATCH] virtio: fix IO request length in virtio SCSI/block #PSBM-78839

[Denis Lunev]

On 10/23/19 5:17 PM, Stefan Hajnoczi wrote:
> On Tue, Oct 22, 2019 at 04:01:57AM +0000, Denis Lunev wrote:
>> On 10/21/19 4:24 PM, Stefan Hajnoczi wrote:
>>> On Fri, Oct 18, 2019 at 02:55:47PM +0300, Denis Plotnikov wrote:
>>>> From: "Denis V. Lunev" <address@hidden>
>>>>
>>>> Linux guests submit IO requests no longer than PAGE_SIZE * max_seg
>>>> field reported by SCSI controler. Thus typical sequential read with
>>>> 1 MB size results in the following pattern of the IO from the guest:
>>>>   8,16   1    15754     2.766095122  2071  D   R 2095104 + 1008 [dd]
>>>>   8,16   1    15755     2.766108785  2071  D   R 2096112 + 1008 [dd]
>>>>   8,16   1    15756     2.766113486  2071  D   R 2097120 + 32 [dd]
>>>>   8,16   1    15757     2.767668961     0  C   R 2095104 + 1008 [0]
>>>>   8,16   1    15758     2.768534315     0  C   R 2096112 + 1008 [0]
>>>>   8,16   1    15759     2.768539782     0  C   R 2097120 + 32 [0]
>>>> The IO was generated by
>>>>   dd if=/dev/sda of=/dev/null bs=1024 iflag=direct
>>>>
>>>> This effectively means that on rotational disks we will observe 3 IOPS
>>>> for each 2 MBs processed. This definitely negatively affects both
>>>> guest and host IO performance.
>>>>
>>>> The cure is relatively simple - we should report lengthy scatter-gather
>>>> ability of the SCSI controller. Fortunately the situation here is very
>>>> good. VirtIO transport layer can accomodate 1024 items in one request
>>>> while we are using only 128. This situation is present since almost
>>>> very beginning. 2 items are dedicated for request metadata thus we
>>>> should publish VIRTQUEUE_MAX_SIZE - 2 as max_seg.
>>>>
>>>> The following pattern is observed after the patch:
>>>>   8,16   1     9921     2.662721340  2063  D   R 2095104 + 1024 [dd]
>>>>   8,16   1     9922     2.662737585  2063  D   R 2096128 + 1024 [dd]
>>>>   8,16   1     9923     2.665188167     0  C   R 2095104 + 1024 [0]
>>>>   8,16   1     9924     2.665198777     0  C   R 2096128 + 1024 [0]
>>>> which is much better.
>>>>
>>>> The dark side of this patch is that we are tweaking guest visible
>>>> parameter, though this should be relatively safe as above transport
>>>> layer support is present in QEMU/host Linux for a very long time.
>>>> The patch adds configurable property for VirtIO SCSI with a new default
>>>> and hardcode option for VirtBlock which does not provide good
>>>> configurable framework.
>>>>
>>>> Unfortunately the commit can not be applied as is. For the real cure we
>>>> need guest to be fixed to accomodate that queue length, which is done
>>>> only in the latest 4.14 kernel. Thus we are going to expose the property
>>>> and tweak it on machine type level.
>>>>
>>>> The problem with the old kernels is that they have
>>>> max_segments <= virtqueue_size restriction which cause the guest
>>>> crashing in the case of violation.
>>>> To fix the case described above in the old kernels we can increase
>>>> virtqueue_size to 256 and max_segments to 254. The pitfall here is
>>>> that seabios allows the virtqueue_size-s < 128, however, the seabios
>>>> patch extending that value to 256 is pending.
>>> If I understand correctly you are relying on Indirect Descriptor support
>>> in the guest driver in order to exceed the Virtqueue Descriptor Table
>>> size.
>>>
>>> Unfortunately the "max_segments <= virtqueue_size restriction" is
>>> required by the VIRTIO 1.1 specification:
>>>
>>>   2.6.5.3.1 Driver Requirements: Indirect Descriptors
>>>
>>>   A driver MUST NOT create a descriptor chain longer than the Queue
>>>   Size of the device.
>>>
>>> So this idea seems to be in violation of the specification?
>>>
>>> There is a bug in hw/block/virtio-blk.c:virtio_blk_update_config() and
>>> hw/scsi/virtio-scsi.c:virtio_scsi_get_config():
>>>
>>>   virtio_stl_p(vdev, &blkcfg.seg_max, 128 - 2);
>>>
>>> This number should be the minimum of blk_get_max_iov() and
>>> virtio_queue_get_num(), minus 2 for the header and footer.
>>>
>>> I looked at the Linux SCSI driver code and it seems each HBA has a
>>> single max_segments number - it does not vary on a per-device basis.
>>> This could be a problem if two host block device with different
>>> max_segments are exposed to the guest through the same virtio-scsi
>>> controller.  Another bug? :(
>>>
>>> Anyway, if you want ~1024 descriptors you should set Queue Size to 1024.
>>> I don't see a spec-compliant way of doing it otherwise.  Hopefully I
>>> have overlooked something and there is a nice way to solve this.
>>>
>>> Stefan
>> you are perfectly correct. We need actually 3 changes to improve
>> guest behavior:
>> 1) This patch, which adds property but does not change anything
>>     useful
> This patch is problematic because it causes existing guest drivers to
> violate the VIRTIO specification (or fail) if the value is set too high.
> In addition, it does not take into account the virtqueue size so the
> default value is too low when the user sets -device ...,queue-size=1024.
>
> Let's calculate blkcfg.seg_max based on the virtqueue size as mentioned
> in my previous email instead.
As far as I understand maximum amount of segments could be more than
virtqueue size for indirect requests (allowed in VirtIO 1.0).

> There is one caveat with my suggestion: drivers are allowed to access
> VIRTIO Configuration Space before virtqueue setup has determined the
> final size.  Therefore the value of this field can change after
> virtqueue setup.  Drivers that set a custom virtqueue size would need to
> read the value after virtqueue setup.  (Linux drivers do not modify the
> virtqueue size so it won't affect them.)
>
> Stefan
I think that we should do that a little bit different :) We can not
change max_segs
just if queue size is changed, this should be somehow bound to machine type.
Thus I propose to add "automatic" value, i.e.

if max_segs is set to 0 the code should set it to queue size -2.
This should be default. Alternatively the value from max_segs should be
taken. Will this work for you?

Please note, currently the specification could also be violated if we will
reduce queue size to 64 :)

Den