[Bug 2025586] Re: Align the iov length to the logical block size

[ChengEn, Du]

** Changed in: qemu
     Assignee: (unassigned) => ChengEn, Du (chengendu)

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https://bugs.launchpad.net/bugs/2025586

Title:
  Align the iov length to the logical block size

Status in QEMU:
  New

Bug description:
  [Impact]
  When the logical block size of the virtual block device is smaller than the 
block device it is backed by on the host,
  qemu encounters a situation where it needs to bounce unaligned buffers during 
the use of direct IO.
  In the past, the logical block size happened to align with the memory page 
offset, leading qemu to mistakenly use the memory offset as the block size.
  However, a kernel commit b1a000d3b8ec resolved this issue by separating 
memory alignment from the logical block size.
  As a result, qemu now has an incorrect understanding of the minimum vector 
size.

  [Fix]
  Upstream commit 25474d90aa50 fixed this issue.
  ==========
  Author:     Keith Busch <kbusch@kernel.org>
  CommitDate: Fri Sep 30 18:43:44 2022 +0200

      block: use the request length for iov alignment

      An iov length needs to be aligned to the logical block size, which may
      be larger than the memory alignment.

      Tested-by: Jens Axboe <axboe@kernel.dk>
      Signed-off-by: Keith Busch <kbusch@kernel.org>
      Message-Id: <20220929200523.3218710-3-kbusch@meta.com>
      Reviewed-by: Kevin Wolf <kwolf@redhat.com>
      Signed-off-by: Kevin Wolf <kwolf@redhat.com>
  ==========

  [Test Plan]
  1. Get a ubuntu image and convert it to RAW format
  wget 
https://cloud-images.ubuntu.com/jammy/current/jammy-server-cloudimg-amd64-disk-kvm.img
  qemu-img convert jammy-server-cloudimg-amd64-disk-kvm.img 
jammy-server-cloudimg-amd64-disk-kvm.raw
  2. Set up a loop device with RAW image
  losetup -b 4096 -f jammy-server-cloudimg-amd64-disk-kvm.raw
  3. Get loop device number by `losetup -a` command
  4. Start the virtual machine
  qemu-system-x86_64 -enable-kvm -drive file=/dev/loopX,format=raw,cache=none 
--nographic

  [Where problems could occur]
  The patch addressed the issue of misusing the memory offset as the block size.
  This problem only occurred when the cache option was set to "none" and the 
Linux kernel being used had the commit b1a000d3b8ec.
  However, it is worth noting that the patch also worked effectively with older 
kernels.

  [Other Info]

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