linux-kernel - Re: [PATCH v2] entry/kvm: Make vCPU tasks exit to userspace when a livepatch is pending

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Date:   Wed, 04 May 2022 09:53:40 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Seth Forshee <sforshee@...italocean.com>,
        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>,
        Jens Axboe <axboe@...nel.dk>,
        Sean Christopherson <seanjc@...gle.com>,
        linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH v2] entry/kvm: Make vCPU tasks exit to userspace when a
 livepatch is pending

Petr Mladek <pmladek@...e.com> writes:

> On Wed 2022-05-04 08:50:22, Seth Forshee wrote:
>> On Wed, May 04, 2022 at 03:07:53PM +0200, Petr Mladek wrote:

>> > If "no" then I do not understand why TIF_NOTIFY_SIGNAL interrupts
>> > the syscall on the real hardware and not in kvm.
>> 
>> It does interrupt, but xfer_to_guest_mode_handle_work() concludes it's
>> not necessary to return to userspace and resumes guest execution.
>
> In this case, we should revert the commit 8df1947c71ee53c7e21
> ("livepatch: Replace the fake signal sending with TIF_NOTIFY_SIGNAL
> infrastructure"). The flag TIF_NOTIFY_SIGNAL clearly does not guarantee
> restarting the syscall or exiting to the user space with -EINTR.
>
> It should solve this problem. And it looks like a cleaner solution
> to me.

Why not just?

diff --git a/kernel/entry/kvm.c b/kernel/entry/kvm.c
index 9d09f489b60e..cbb192aec13a 100644
--- a/kernel/entry/kvm.c
+++ b/kernel/entry/kvm.c
@@ -8,13 +8,7 @@ static int xfer_to_guest_mode_work(struct kvm_vcpu *vcpu, unsigned long ti_work)
        do {
                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) {
+               if (ti_work & (_TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL)) {
                        kvm_handle_signal_exit(vcpu);
                        return -EINTR;
                }

As far as I can tell the only reason _TIF_NOTIFY_SIGNAL was handled any
differently than _TIF_SIGPENDING in xfer_to_guest_mode_work is because
of historical confusion.

Eric