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Re: [PATCH] virtio: fix IO request length in virtio SCSI/block #PSBM-788
From: |
Stefan Hajnoczi |
Subject: |
Re: [PATCH] virtio: fix IO request length in virtio SCSI/block #PSBM-78839 |
Date: |
Mon, 21 Oct 2019 14:24:55 +0100 |
User-agent: |
Mutt/1.12.1 (2019-06-15) |
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
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