On 5/4/22 12:08 PM, Seth Forshee wrote: > 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(). Reviewed-by: Jens Axboe <axboe@xxxxxxxxx> -- Jens Axboe
