linux-kernel - Re: [PATCH] entry/kvm: Exit to user mode when TIF_NOTIFY

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Date:   Thu, 5 May 2022 09:30:20 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Seth Forshee <sforshee@...italocean.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org, kvm@...r.kernel.org,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH] entry/kvm: Exit to user mode when TIF_NOTIFY_SIGNAL is
 set

On Wed 2022-05-04 13:08:40, 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@...ssion.com>
> Cc: Petr Mladek <pmladek@...e.com>
> Signed-off-by: Seth Forshee <sforshee@...italocean.com>

Acked-by: Petr Mladek <pmladek@...e.com>

Thanks Seth for discovering the problem.
Thanks everyone who helped to find the right solution.

Best Regards.
Petr