[PATCH 0/3] kvm "fake DAX" device

Pankaj Gupta
2018-08-31T13:30:15Z

This patch series has implementation for "fake DAX".
"fake DAX" 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 driver and qemu device changes in separate
patch sets for easy review and it has been tested together.

Details of project idea for 'fake DAX' flushing interface
is shared [2] & [3].

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[4] 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 RFC v3: [1]
- 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 (3):
nd: move nd_region to common header
libnvdimm: nd_region flush callback support
virtio-pmem: Add virtio-pmem guest driver

[1] https://lkml.org/lkml/2018/7/13/102
[2] https://www.spinics.net/lists/kvm/msg149761.html
[3] https://www.spinics.net/lists/kvm/msg153095.html
[4] https://marc.info/?l=qemu-devel&m=153555721901824&w=2

drivers/acpi/nfit/core.c | 7 -
drivers/nvdimm/claim.c | 3
drivers/nvdimm/nd.h | 39 -----
drivers/nvdimm/pmem.c | 12 +
drivers/nvdimm/region_devs.c | 12 +
drivers/virtio/Kconfig | 9 +
drivers/virtio/Makefile | 1
drivers/virtio/virtio_pmem.c | 255 +++++++++++++++++++++++++++++++++++++++
include/linux/libnvdimm.h | 4
include/linux/nd.h | 40 ++++++
include/uapi/linux/virtio_ids.h | 1
include/uapi/linux/virtio_pmem.h | 40 ++++++
12 files changed, 374 insertions(+), 49 deletions(-)