Commit 3e684903 authored by Seth Forshee's avatar Seth Forshee Committed by Paolo Bonzini
Browse files

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



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: default avatar"Eric W. Biederman" <ebiederm@xmission.com>
Cc: Petr Mladek <pmladek@suse.com>
Signed-off-by: default avatarSeth Forshee <sforshee@digitalocean.com>
Message-Id: <20220504180840.2907296-1-sforshee@digitalocean.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent e8bc2427

kernel/entry/kvm.c

+0 −6
Original line number Diff line number Diff line
@@ -9,12 +9,6 @@ static int xfer_to_guest_mode_work(struct kvm_vcpu *vcpu, unsigned long ti_work) 
		int ret;



		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);

			return -EINTR;

		}