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entry/kvm: Exit to user mode when TIF_NOTIFY_SIGNAL is set

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A livepatch transition may stall indefinitely when a kvm vCPU is heavily
loaded. To the host, the vCPU task is a user thread which is spending a
very long time in the ioctl(KVM_RUN) syscall. During livepatch
transition, set_notify_signal() will be called on such tasks to
interrupt the syscall so that the task can be transitioned. This
interrupts guest execution, but when xfer_to_guest_mode_work() sees that
TIF_NOTIFY_SIGNAL is set but not TIF_SIGPENDING it concludes that an
exit to user mode is unnecessary, and guest execution is resumed without
transitioning the task for the livepatch.

This handling of TIF_NOTIFY_SIGNAL is incorrect, as set_notify_signal()
is expected to break tasks out of interruptible kernel loops and cause
them to return to userspace. Change xfer_to_guest_mode_work() to handle
TIF_NOTIFY_SIGNAL the same as TIF_SIGPENDING, signaling to the vCPU run
loop that an exit to userpsace is needed. Any pending task_work will be
run when get_signal() is called from exit_to_user_mode_loop(), so there
is no longer any need to run task work from xfer_to_guest_mode_work().

Suggested-by: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Petr Mladek <pmladek@suse.com>
Signed-off-by: Seth Forshee <sforshee@digitalocean.com>
Message-Id: <20220504180840.2907296-1-sforshee@digitalocean.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Seth Forshee 2022-05-04 13:08:40 -05:00 committed by Paolo Bonzini
parent e8bc242701
commit 3e684903a8
1 changed files with 0 additions and 6 deletions
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6
kernel/entry/kvm.c
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@ -9,12 +9,6 @@ static int xfer_to_guest_mode_work(struct kvm_vcpu *vcpu, unsigned long ti_work)
int ret; int ret;
if (ti_work & (_TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL)) { if (ti_work & (_TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL)) {
clear_notify_signal();
if (task_work_pending(current))
task_work_run();
}
if (ti_work & _TIF_SIGPENDING) {
kvm_handle_signal_exit(vcpu); kvm_handle_signal_exit(vcpu);
return -EINTR; return -EINTR;
} }