[Qemu-devel] [RFC/PATCHSET 0/3] virtio-pstore: Implement virtio pstore device

[Namhyung Kim]

Hello,

This patchset is a proof of concept of virtio-pstore idea [1].  It has
some rough edges and I'm not familiar with this area, so please give
me feedbacks and advices if I'm going to a wrong direction.

It started from the fact that dumping ftrace buffer at kernel
oops/panic takes too much time.  Although there's a way to reduce the
size of the original data, sometimes I want to have the information as
many as possible.  Maybe kexec/kdump can solve this problem but it
consumes some portion of guest memory so I'd like to avoid it.  And I
know the qemu + crashtool can dump and analyze the whole guest memory
including the ftrace buffer without wasting guest memory, but it adds
one more layer and has some limitation as an out-of-tree tool like not
being in sync with the kernel changes.

So I think it'd be great using the pstore interface to dump guest
kernel data on the host.  One can read the data on the host directly
or on the guest (at the next boot) using pstore filesystem as usual.
While this patchset only implements dumping kernel log buffer, it can
be extended to have ftrace buffer and probably some more..

The patch 0001 implements virtio pstore driver.  It has a single virt
queue, pstore buffer and header structure.  The virtio_pstore_hdr
struct is to give information about the current pstore operation.

The patch 0002 and 0003 implement virtio-pstore legacy PCI device on
qemu-kvm and kvmtool respectively.  I referenced virtio-baloon and
virtio-rng implementations and I don't know whether kvmtool supports
modern virtio 1.0+ spec.

For example, using virtio-pstore on qemu looks like below:

  $ qemu-system-x86_64 -enable-kvm -device virtio-pstore,directory=xxx

When guest kernel gets panic the log messages will be saved under the
xxx directory.

  $ ls xxx
  dmesg-0.enc.z  dmesg-1.enc.z

As you can see the pstore subsystem compresses the log data using
zlib.  The data can be extracted with the following command:

  $ cat xxx/dmesg-0.enc.z | \
  > python -c 'import sys, zlib; print(zlib.decompress(sys.stdin.read()))'
  Oops#1 Part1
  <5>[    0.000000] Linux version 4.6.0kvm+ (address@hidden) (gcc version 5.3.0 
(GCC) ) #145 SMP Mon Jul 18 10:22:45 KST 2016
  <6>[    0.000000] Command line: root=/dev/vda console=ttyS0
  <6>[    0.000000] x86/fpu: Legacy x87 FPU detected.
  <6>[    0.000000] x86/fpu: Using 'eager' FPU context switches.
  <6>[    0.000000] e820: BIOS-provided physical RAM map:
  <6>[    0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] 
usable
  <6>[    0.000000] BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] 
reserved
  <6>[    0.000000] BIOS-e820: [mem 0x00000000000f0000-0x00000000000fffff] 
reserved
  <6>[    0.000000] BIOS-e820: [mem 0x0000000000100000-0x0000000007fddfff] 
usable
  <6>[    0.000000] BIOS-e820: [mem 0x0000000007fde000-0x0000000007ffffff] 
reserved
  <6>[    0.000000] BIOS-e820: [mem 0x00000000feffc000-0x00000000feffffff] 
reserved
  <6>[    0.000000] BIOS-e820: [mem 0x00000000fffc0000-0x00000000ffffffff] 
reserved
  <6>[    0.000000] NX (Execute Disable) protection: active
  <6>[    0.000000] SMBIOS 2.8 present.
  <7>[    0.000000] DMI: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 
rel-1.9.3-0-ge2fc41e-prebuilt.qemu-project.org 04/01/2014
  ...

Maybe we can add a config option to control the compression later.


Cc: Paolo Bonzini <address@hidden>
Cc: Radim Krčmář <address@hidden>
Cc: "Michael S. Tsirkin" <address@hidden>
Cc: Anthony Liguori <address@hidden>
Cc: Anton Vorontsov <address@hidden>
Cc: Colin Cross <address@hidden>
Cc: Kees Cook <address@hidden>
Cc: Tony Luck <address@hidden>
Cc: Steven Rostedt <address@hidden>
Cc: Ingo Molnar <address@hidden>
Cc: Minchan Kim <address@hidden>
Cc: address@hidden
Cc: address@hidden
Cc: address@hidden

[1] https://lkml.org/lkml/2016/7/1/6


Thanks,
Namhyung