Re: [Qemu-devel] [PATCH v3 0/5] kvm "virtio pmem" device

[Pankaj Gupta]

Please ignore this series as my network went down while 
sending this. I will send this series again.

Thanks,
Pankaj  

> 
> This patch series has implementation for "virtio pmem".
>  "virtio pmem" is fake persistent memory(nvdimm) in guest
>  which allows to bypass the guest page cache. This also
>  implements a VIRTIO based asynchronous flush mechanism.
>  
>  Sharing guest kernel driver in this patchset with the
>  changes suggested in v2. Tested with Qemu side device
>  emulation for virtio-pmem [6].
>  
>  Details of project idea for 'virtio pmem' flushing interface
>  is shared [3] & [4].
> 
>  Implementation is divided into two parts:
>  New virtio pmem guest driver and qemu code changes for new
>  virtio pmem paravirtualized device.
> 
> 1. Guest virtio-pmem kernel driver
> ---------------------------------
>    - Reads persistent memory range from paravirt device and
>      registers with 'nvdimm_bus'.
>    - 'nvdimm/pmem' driver uses this information to allocate
>      persistent memory region and setup filesystem operations
>      to the allocated memory.
>    - virtio pmem driver implements asynchronous flushing
>      interface to flush from guest to host.
> 
> 2. Qemu virtio-pmem device
> ---------------------------------
>    - Creates virtio pmem device and exposes a memory range to
>      KVM guest.
>    - At host side this is file backed memory which acts as
>      persistent memory.
>    - Qemu side flush uses aio thread pool API's and virtio
>      for asynchronous guest multi request handling.
> 
>    David Hildenbrand CCed also posted a modified version[6] of
>    qemu virtio-pmem code based on updated Qemu memory device API.
> 
>  Virtio-pmem errors handling:
>  ----------------------------------------
>   Checked behaviour of virtio-pmem for below types of errors
>   Need suggestions on expected behaviour for handling these errors?
> 
>   - Hardware Errors: Uncorrectable recoverable Errors:
>   a] virtio-pmem:
>     - As per current logic if error page belongs to Qemu process,
>       host MCE handler isolates(hwpoison) that page and send SIGBUS.
>       Qemu SIGBUS handler injects exception to KVM guest.
>     - KVM guest then isolates the page and send SIGBUS to guest
>       userspace process which has mapped the page.
>   
>   b] Existing implementation for ACPI pmem driver:
>     - Handles such errors with MCE notifier and creates a list
>       of bad blocks. Read/direct access DAX operation return EIO
>       if accessed memory page fall in bad block list.
>     - It also starts backgound scrubbing.
>     - Similar functionality can be reused in virtio-pmem with MCE
>       notifier but without scrubbing(no ACPI/ARS)? Need inputs to
>       confirm if this behaviour is ok or needs any change?
> 
> Changes from PATCH v2: [1]
> - Disable MAP_SYNC for ext4 & XFS filesystems - [Dan]
> - Use name 'virtio pmem' in place of 'fake dax'
> 
> Changes from PATCH v1: [2]
> - 0-day build test for build dependency on libnvdimm
> 
>  Changes suggested by - [Dan Williams]
> - Split the driver into two parts virtio & pmem
> - Move queuing of async block request to block layer
> - Add "sync" parameter in nvdimm_flush function
> - Use indirect call for nvdimm_flush
> - Donât move declarations to common global header e.g nd.h
> - nvdimm_flush() return 0 or -EIO if it fails
> - Teach nsio_rw_bytes() that the flush can fail
> - Rename nvdimm_flush() to generic_nvdimm_flush()
> - Use 'nd_region->provider_data' for long dereferencing
> - Remove virtio_pmem_freeze/restore functions
> - Remove BSD license text with SPDX license text
> 
> - Add might_sleep() in virtio_pmem_flush - [Luiz]
> - Make spin_lock_irqsave() narrow
> 
> Changes from RFC v3
> - Rebase to latest upstream - Luiz
> - Call ndregion->flush in place of nvdimm_flush- Luiz
> - kmalloc return check - Luiz
> - virtqueue full handling - Stefan
> - Don't map entire virtio_pmem_req to device - Stefan
> - request leak, correct sizeof req- Stefan
> - Move declaration to virtio_pmem.c
> 
> Changes from RFC v2:
> - Add flush function in the nd_region in place of switching
>   on a flag - Dan & Stefan
> - Add flush completion function with proper locking and wait
>   for host side flush completion - Stefan & Dan
> - Keep userspace API in uapi header file - Stefan, MST
> - Use LE fields & New device id - MST
> - Indentation & spacing suggestions - MST & Eric
> - Remove extra header files & add licensing - Stefan
> 
> Changes from RFC v1:
> - Reuse existing 'pmem' code for registering persistent
>   memory and other operations instead of creating an entirely
>   new block driver.
> - Use VIRTIO driver to register memory information with
>   nvdimm_bus and create region_type accordingly.
> - Call VIRTIO flush from existing pmem driver.
> 
> Pankaj Gupta (5):
>    libnvdimm: nd_region flush callback support
>    virtio-pmem: Add virtio-pmem guest driver
>    libnvdimm: add nd_region buffered dax_dev flag
>    ext4: disable map_sync for virtio pmem
>    xfs: disable map_sync for virtio pmem
> 
> [2] https://lkml.org/lkml/2018/8/31/407
> [3] https://www.spinics.net/lists/kvm/msg149761.html
> [4] https://www.spinics.net/lists/kvm/msg153095.html
> [5] https://lkml.org/lkml/2018/8/31/413
> [6] https://marc.info/?l=qemu-devel&m=153555721901824&w=2
> 
>  drivers/acpi/nfit/core.c         |    4 -
>  drivers/dax/super.c              |   17 +++++
>  drivers/nvdimm/claim.c           |    6 +
>  drivers/nvdimm/nd.h              |    1
>  drivers/nvdimm/pmem.c            |   15 +++-
>  drivers/nvdimm/region_devs.c     |   45 +++++++++++++-
>  drivers/nvdimm/virtio_pmem.c     |   84 ++++++++++++++++++++++++++
>  drivers/virtio/Kconfig           |   10 +++
>  drivers/virtio/Makefile          |    1
>  drivers/virtio/pmem.c            |  125
>  +++++++++++++++++++++++++++++++++++++++
>  fs/ext4/file.c                   |   11 +++
>  fs/xfs/xfs_file.c                |    8 ++
>  include/linux/dax.h              |    9 ++
>  include/linux/libnvdimm.h        |   11 +++
>  include/linux/virtio_pmem.h      |   60 ++++++++++++++++++
>  include/uapi/linux/virtio_ids.h  |    1
>  include/uapi/linux/virtio_pmem.h |   10 +++
>  17 files changed, 406 insertions(+), 12 deletions(-)
> 
> 
>