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

[Michael S. Tsirkin]

On Mon, Oct 21, 2019 at 02:24:55PM +0100, 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



We can extend the spec of course. And we can also
have different vq sizes between legacy and modern
interfaces.

-- 
MST