A Linux guest will perform IRQ migration after the IRQ has happened,
updating the RTE to point to the new destination CPU and then unmasking
the interrupt.
However, when the guest updates the RTE, ioapic_mem_write() calls
ioapic_service(), which redelivers the pending level interrupt via
kvm_set_irq(), *before* calling ioapic_update_kvm_routes() which sets
the new target CPU.
Thus, the IRQ which is supposed to go to the new target CPU is instead
misdelivered to the previous target. An example where the guest kernel
is attempting to migrate from CPU#2 to CPU#0 shows:
xenstore_read tx 0 path control/platform-feature-xs_reset_watches
ioapic_set_irq vector: 11 level: 1
ioapic_set_remote_irr set remote irr for pin 11
ioapic_service: trigger KVM IRQ 11
[ 0.523627] The affinity mask was 0-3 and the handler is on 2
ioapic_mem_write ioapic mem write addr 0x0 regsel: 0x27 size 0x4 val 0x26
ioapic_update_kvm_routes: update KVM route for IRQ 11: fee02000 8021
ioapic_mem_write ioapic mem write addr 0x10 regsel: 0x26 size 0x4 val 0x18021
xenstore_reset_watches
ioapic_set_irq vector: 11 level: 1
ioapic_mem_read ioapic mem read addr 0x10 regsel: 0x26 size 0x4 retval 0x1c021
[ 0.524569] ioapic_ack_level IRQ 11 moveit = 1
ioapic_eoi_broadcast EOI broadcast for vector 33
ioapic_clear_remote_irr clear remote irr for pin 11 vector 33
ioapic_mem_write ioapic mem write addr 0x0 regsel: 0x26 size 0x4 val 0x26
ioapic_mem_read ioapic mem read addr 0x10 regsel: 0x26 size 0x4 retval 0x18021
[ 0.525235] ioapic_finish_move IRQ 11 calls irq_move_masked_irq()
[ 0.526147] irq_do_set_affinity for IRQ 11, 0
[ 0.526732] ioapic_set_affinity for IRQ 11, 0
[ 0.527330] ioapic_setup_msg_from_msi for IRQ11 target 0
ioapic_mem_write ioapic mem write addr 0x0 regsel: 0x26 size 0x4 val 0x27
ioapic_mem_write ioapic mem write addr 0x10 regsel: 0x27 size 0x4 val 0x0
ioapic_mem_write ioapic mem write addr 0x0 regsel: 0x27 size 0x4 val 0x26
ioapic_mem_write ioapic mem write addr 0x10 regsel: 0x26 size 0x4 val 0x18021
[ 0.527623] ioapic_set_affinity returns 0
[ 0.527623] ioapic_finish_move IRQ 11 calls unmask_ioapic_irq()
ioapic_mem_write ioapic mem write addr 0x0 regsel: 0x26 size 0x4 val 0x26
ioapic_mem_write ioapic mem write addr 0x10 regsel: 0x26 size 0x4 val 0x8021
ioapic_set_remote_irr set remote irr for pin 11
ioapic_service: trigger KVM IRQ 11
ioapic_update_kvm_routes: update KVM route for IRQ 11: fee00000 8021
[ 0.529571] The affinity mask was 0 and the handler is on 2
[ xenstore_watch path memory/target token FFFFFFFF92847D40
There are no other code paths in ioapic_mem_write() which need the KVM
IRQ routing table to be updated, so just shift the call from the end
of the function to happen right before the call to ioapic_service()
and thus deliver the re-enabled IRQ to the right place.
Alternative fixes might have been just to remove the part in
ioapic_service() which delivers the IRQ via kvm_set_irq() because
surely delivering as MSI ought to work just fine anyway in all cases?
That code lacks a comment justifying its existence.
Or maybe in the specific case shown in the above log, it would have
sufficed for ioapic_update_kvm_routes() to update the route *even*
when the IRQ is masked. It's not like it's actually going to get
triggered unless QEMU deliberately does so, anyway? But that only
works because the target CPU happens to be in the high word of the
RTE; if something in the *low* word (vector, perhaps) was changed
at the same time as the unmask, we'd still trigger with stale data.
Fixes: 15eafc2e60 "kvm: x86: add support for KVM_CAP_SPLIT_IRQCHIP"
Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Reviewed-by: Peter Xu <peterx@redhat.com>
Message-Id: <20230308111952.2728440-2-dwmw2@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
===========
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:
.. code-block:: shell
mkdir build
cd build
../configure
make
Additional information can also be found online via the QEMU website:
* `<https://wiki.qemu.org/Hosts/Linux>`_
* `<https://wiki.qemu.org/Hosts/Mac>`_
* `<https://wiki.qemu.org/Hosts/W32>`_
Submitting patches
==================
The QEMU source code is maintained under the GIT version control system.
.. code-block:: shell
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
* `<https://wiki.qemu.org/Contribute/SubmitAPatch>`_
* `<https://wiki.qemu.org/Contribute/TrivialPatches>`_
The QEMU website is also maintained under source control.
.. code-block:: shell
git clone https://gitlab.com/qemu-project/qemu-web.git
* `<https://www.qemu.org/2017/02/04/the-new-qemu-website-is-up/>`_
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
* `<https://github.com/stefanha/git-publish>`_
The workflow with 'git-publish' is:
.. code-block:: shell
$ 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:
.. code-block:: shell
$ 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:
* `<https://gitlab.com/qemu-project/qemu/-/issues>`_
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:
* `<https://wiki.qemu.org/Contribute/ReportABug>`_
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
* `<mailto:qemu-devel@nongnu.org>`_
* `<https://lists.nongnu.org/mailman/listinfo/qemu-devel>`_
* #qemu on irc.oftc.net
Information on additional methods of contacting the community can be
found online via the QEMU website:
* `<https://wiki.qemu.org/Contribute/StartHere>`_