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From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     Petr Mladek <pmladek@suse.com>
Cc:     Seth Forshee <sforshee@digitalocean.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Jiri Kosina <jikos@kernel.org>,
        Miroslav Benes <mbenes@suse.cz>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Jens Axboe <axboe@kernel.dk>,
        Sean Christopherson <seanjc@google.com>,
        linux-kernel@vger.kernel.org, live-patching@vger.kernel.org,
        kvm@vger.kernel.org
References: <20220503174934.2641605-1-sforshee@digitalocean.com>
        <20220504130753.GB8069@pathway.suse.cz>
        <YnKEnqfxSyVmSGYx@do-x1extreme>
        <20220504142809.GC8069@pathway.suse.cz>
Date:   Wed, 04 May 2022 09:53:40 -0500
In-Reply-To: <20220504142809.GC8069@pathway.suse.cz> (Petr Mladek's message of
        "Wed, 4 May 2022 16:28:09 +0200")
Message-ID: <87ee19fix7.fsf@email.froward.int.ebiederm.org>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux)
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Subject: Re: [PATCH v2] entry/kvm: Make vCPU tasks exit to userspace when a
 livepatch is pending
Precedence: bulk

Petr Mladek <pmladek@suse.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