Re: [PATCH] entry/kvm: Exit to user mode when TIF_NOTIFY_SIGNAL is set

[Paolo Bonzini]

On 6/6/22 16:13, Seth Forshee wrote:
> On Wed, May 04, 2022 at 01:08:40PM -0500, 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().
> >
> > Suggested-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> > Cc: Petr Mladek <pmladek@xxxxxxxx>
> > Signed-off-by: Seth Forshee <sforshee@xxxxxxxxxxxxxxxx>
>
> Friendly reminder as it seems like this patch may have been forgotten.

Probably AB-BA maintainer deadlock.  I have queued it now.

Paolo

> Thanks,
> Seth
>
> > ---
> >   kernel/entry/kvm.c | 6 ------
> >   1 file changed, 6 deletions(-)
> >
> > diff --git a/kernel/entry/kvm.c b/kernel/entry/kvm.c
> > index 9d09f489b60e..2e0f75bcb7fd 100644
> > --- a/kernel/entry/kvm.c
> > +++ b/kernel/entry/kvm.c
> > @@ -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;
> >   		}
> > --
> > 2.32.0