The purpose of the KVM_SET_SIGNAL_MASK API is to let userspace "kick"
a VCPU out of KVM_RUN through a POSIX signal. A signal is attached
to a dummy signal handler; by blocking the signal outside KVM_RUN and
unblocking it inside, this possible race is closed:
VCPU thread service thread
--------------------------------------------------------------
check flag
set flag
raise signal
(signal handler does nothing)
KVM_RUN
However, one issue with KVM_SET_SIGNAL_MASK is that it has to take
tsk->sighand->siglock on every KVM_RUN. This lock is often on a
remote NUMA node, because it is on the node of a thread's creator.
Taking this lock can be very expensive if there are many userspace
exits (as is the case for SMP Windows VMs without Hyper-V reference
time counter).
As an alternative, we can put the flag directly in kvm_run so that
KVM can see it:
VCPU thread service thread
--------------------------------------------------------------
raise signal
signal handler
set run->immediate_exit
KVM_RUN
check run->immediate_exit
This is what the last patch in this series does, together with
the corresponding kernel API. The first six patches are a long
detour in the signal handling code, moving KVM-specific stuff
from cpus.c to kvm-all.c so that we have a better hook point for
KVM_CAP_IMMEDIATE_EXIT (patches 1-3, 6).
Because KVM_SET_SIGNAL_MASK is also unblocking SIGBUS so that
BUS_MCEERR_AR actions can be delivered via sigwait, we also have
to rewrite it (patch 4-5) to avoid sigwait, stowing the machine check
exception as soon as KVM_RUN exits and process it outside the signal
handler.
The seventh patch would of course be split between a linux-headers
update and the rest.
Paolo
ps: As an aside, I finally figured out how to test machine check
forwarding and I hope to write something about it.
Paolo Bonzini (7):
cpus: remove ugly cast on sigbus_handler
KVM: x86: cleanup SIGBUS handlers
cpus: reorganize signal handling code
KVM: remove kvm_arch_on_sigbus
KVM: do not use sigtimedwait to catch SIGBUS
KVM: move SIG_IPI handling to kvm-all.c
KVM: use KVM_CAP_IMMEDIATE_EXIT
cpus.c | 102 ++++++------------------------------
include/qemu/compatfd.h | 42 ---------------
include/qemu/osdep.h | 35 +++++++++++++
include/sysemu/kvm.h | 11 ++--
kvm-all.c | 130 +++++++++++++++++++++++++++++++++++++++++++---
kvm-stub.c | 12 ++---
linux-headers/linux/kvm.h | 4 +-
main-loop.c | 5 +-
os-win32.c | 7 +++
target/arm/kvm.c | 10 ----
target/i386/kvm.c | 81 ++++++++---------------------
target/mips/kvm.c | 12 -----
target/ppc/kvm.c | 10 ----
target/s390x/kvm.c | 10 ----
util/compatfd.c | 1 -
util/oslib-posix.c | 33 ++++++++++++
16 files changed, 249 insertions(+), 256 deletions(-)
delete mode 100644 include/qemu/compatfd.h
--
1.8.3.1
