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

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Message-Id: <20220503125729.2556498-1-sforshee@digitalocean.com>
Date:   Tue,  3 May 2022 07:57:29 -0500
From:   Seth Forshee <sforshee@...italocean.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Petr Mladek <pmladek@...e.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
        kvm@...r.kernel.org
Subject: [PATCH] entry/kvm: Make vCPU tasks exit to userspace when a livepatch is pending

A livepatch migration for a task can only happen when the task is
sleeping or it exits to userspace. This may happen infrequently for a
heavily loaded vCPU task, leading to livepatch transition failures.

Fake signals will be sent to tasks which fail to migrate via stack
checking. This will cause running vCPU tasks to exit guest mode, but
since no signal is pending they return to guest execution without
exiting to userspace. Fix this by treating a pending livepatch migration
like a pending signal, exiting to userspace with EINTR. This allows the
migration to complete, and userspace should re-excecute KVM_RUN to
resume guest execution.

In my testing, systems where livepatching would timeout after 60 seconds
were able to load livepatches within a couple of seconds with this
change.

Signed-off-by: Seth Forshee <sforshee@...italocean.com>
---
 kernel/entry/kvm.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/entry/kvm.c b/kernel/entry/kvm.c
index 9d09f489b60e..efe4b791c253 100644
--- a/kernel/entry/kvm.c
+++ b/kernel/entry/kvm.c
@@ -14,7 +14,12 @@ static int xfer_to_guest_mode_work(struct kvm_vcpu *vcpu, unsigned long ti_work)
 				task_work_run();
 		}

-		if (ti_work & _TIF_SIGPENDING) {
+		/*
+		 * When a livepatch migration is pending, force an exit to
+		 * userspace as though a signal is pending to allow the
+		 * migration to complete.
+		 */
+		if (ti_work & (_TIF_SIGPENDING | _TIF_PATCH_PENDING)) {
 			kvm_handle_signal_exit(vcpu);
 			return -EINTR;
 		}
-- 
2.32.0