Skip to content
Snippets Groups Projects
Select Git revision
  • feature/libtcg-rebase default protected
1 result
Compare
user avatar
virtio-mem: Support VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE
David Hildenbrand authored 
With VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE, we signal the VM that reading
unplugged memory is not supported. We have to fail feature negotiation
in case the guest does not support VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE.

First, VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE is required to properly handle
memory backends (or architectures) without support for the shared zeropage
in the hypervisor cleanly. Without the shared zeropage, even reading an
unpopulated virtual memory location can populate real memory and
consequently consume memory in the hypervisor. We have a guaranteed shared
zeropage only on MAP_PRIVATE anonymous memory.

Second, we want VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE to be the default
long-term as even populating the shared zeropage can be problematic: for
example, without THP support (possible) or without support for the shared
huge zeropage with THP (unlikely), the PTE page tables to hold the shared
zeropage entries can consume quite some memory that cannot be reclaimed
easily.

Third, there are other optimizations+features (e.g., protection of
unplugged memory, reducing the total memory slot size and bitmap sizes)
that will require VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE.

We really only support x86 targets with virtio-mem for now (and
Linux similarly only support x86), but that might change soon, so prepare
for different targets already.

Add a new "unplugged-inaccessible" tristate property for x86 targets:
- "off" will keep VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE unset and legacy
  guests working.
- "on" will set VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE and stop legacy guests
  from using the device.
- "auto" selects the default based on support for the shared zeropage.

Warn in case the property is set to "off" and we don't have support for the
shared zeropage.

For existing compat machines, the property will default to "off", to
not change the behavior but eventually warn about a problematic setup.
Short-term, we'll set the property default to "auto" for new QEMU machines.
Mid-term, we'll set the property default to "on" for new QEMU machines.
Long-term, we'll deprecate the parameter and disallow legacy
guests completely.

The property has to match on the migration source and destination. "auto"
will result in the same VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE setting as long
as the qemu command line (esp. memdev) match -- so "auto" is good enough
for migration purposes and the parameter doesn't have to be migrated
explicitly.

Reviewed-by: default avatarMichal Privoznik <mprivozn@redhat.com>
Signed-off-by: default avatarDavid Hildenbrand <david@redhat.com>
Message-Id: <20211217134039.29670-3-david@redhat.com>
Reviewed-by: default avatarMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: default avatarMichael S. Tsirkin <mst@redhat.com>
23ad8dec
History
user avatar 23ad8dec
History
Name Last commit Last update
.github/workflows
.gitlab-ci.d
.gitlab/issue_templates
accel
audio
authz
backends
block
bsd-user
chardev
common-user
configs
contrib
crypto
disas
docs
dump
ebpf
fpu
fsdev
gdb-xml
hw
include
io
libdecnumber
linux-headers
linux-user
migration
monitor
nbd
net
pc-bios
plugins
po
python
qapi
qga
qobject
qom
replay
roms
scripts
scsi
semihosting
softmmu
storage-daemon
stubs
subprojects/libvhost-user
target
tcg
tests
tools
trace
ui
util
.cirrus.yml
.dir-locals.el
.editorconfig
.exrc
.gdbinit
.gitattributes
.gitignore
.gitlab-ci.yml
.gitmodules
.gitpublish
.mailmap
.patchew.yml
.readthedocs.yml
.travis.yml
COPYING
COPYING.LIB
Kconfig
Kconfig.host
LICENSE
MAINTAINERS
Makefile
README.rst
VERSION
block.c
blockdev-nbd.c
blockdev.c
blockjob.c
configure
cpu.c
cpus-common.c
disas.c
gdbstub.c
gitdm.config
hmp-commands-info.hx
hmp-commands.hx
iothread.c
job-qmp.c
job.c
memory_ldst.c.inc
meson.build
meson_options.txt
module-common.c
os-posix.c
os-win32.c
page-vary-common.c
README.rst

QEMU README

QEMU is a generic and open source machine & userspace emulator and virtualizer.

QEMU is capable of emulating a complete machine in software without any need for hardware virtualization support. By using dynamic translation, it achieves very good performance. QEMU can also integrate with the Xen and KVM hypervisors to provide emulated hardware while allowing the hypervisor to manage the CPU. With hypervisor support, QEMU can achieve near native performance for CPUs. When QEMU emulates CPUs directly it is capable of running operating systems made for one machine (e.g. an ARMv7 board) on a different machine (e.g. an x86_64 PC board).

QEMU is also capable of providing userspace API virtualization for Linux and BSD kernel interfaces. This allows binaries compiled against one architecture ABI (e.g. the Linux PPC64 ABI) to be run on a host using a different architecture ABI (e.g. the Linux x86_64 ABI). This does not involve any hardware emulation, simply CPU and syscall emulation.

QEMU aims to fit into a variety of use cases. It can be invoked directly by users wishing to have full control over its behaviour and settings. It also aims to facilitate integration into higher level management layers, by providing a stable command line interface and monitor API. It is commonly invoked indirectly via the libvirt library when using open source applications such as oVirt, OpenStack and virt-manager.

QEMU as a whole is released under the GNU General Public License, version 2. For full licensing details, consult the LICENSE file.

Documentation

Documentation can be found hosted online at https://www.qemu.org/documentation/. The documentation for the current development version that is available at https://www.qemu.org/docs/master/ is generated from the docs/ folder in the source tree, and is built by Sphinx <https://www.sphinx-doc.org/en/master/>_.

Building

QEMU is multi-platform software intended to be buildable on all modern Linux platforms, OS-X, Win32 (via the Mingw64 toolchain) and a variety of other UNIX targets. The simple steps to build QEMU are:

mkdir build
cd build
../configure
make

Additional information can also be found online via the QEMU website:

Submitting patches

The QEMU source code is maintained under the GIT version control system.

git clone https://gitlab.com/qemu-project/qemu.git

When submitting patches, one common approach is to use 'git format-patch' and/or 'git send-email' to format & send the mail to the qemu-devel@nongnu.org mailing list. All patches submitted must contain a 'Signed-off-by' line from the author. Patches should follow the guidelines set out in the style section <https://www.qemu.org/docs/master/devel/style.html> of the Developers Guide.

Additional information on submitting patches can be found online via the QEMU website

The QEMU website is also maintained under source control.

git clone https://gitlab.com/qemu-project/qemu-web.git

A 'git-publish' utility was created to make above process less cumbersome, and is highly recommended for making regular contributions, or even just for sending consecutive patch series revisions. It also requires a working 'git send-email' setup, and by default doesn't automate everything, so you may want to go through the above steps manually for once.

For installation instructions, please go to

The workflow with 'git-publish' is:

$ git checkout master -b my-feature
$ # work on new commits, add your 'Signed-off-by' lines to each
$ git publish

Your patch series will be sent and tagged as my-feature-v1 if you need to refer back to it in the future.

Sending v2:

$ git checkout my-feature # same topic branch
$ # making changes to the commits (using 'git rebase', for example)
$ git publish

Your patch series will be sent with 'v2' tag in the subject and the git tip will be tagged as my-feature-v2.

Bug reporting

The QEMU project uses GitLab issues to track bugs. Bugs found when running code built from QEMU git or upstream released sources should be reported via:

If using QEMU via an operating system vendor pre-built binary package, it is preferable to report bugs to the vendor's own bug tracker first. If the bug is also known to affect latest upstream code, it can also be reported via GitLab.

For additional information on bug reporting consult:

ChangeLog

For version history and release notes, please visit https://wiki.qemu.org/ChangeLog/ or look at the git history for more detailed information.

Contact

The QEMU community can be contacted in a number of ways, with the two main methods being email and IRC

Information on additional methods of contacting the community can be found online via the QEMU website: