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

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Date:   Tue, 3 May 2022 12:53:56 -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>,
        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

On Tue, May 03, 2022 at 12:49:34PM -0500, Seth Forshee wrote:
> A task can be livepatched only when it 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 patching 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
> task to be patched, 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>
> ---
> Changes in v2:
>  - Added _TIF_SIGPENDING to XFER_TO_GUEST_MODE_WORK

Clearly I meant _TIF_PATCH_PENDING here and not _TIF_SIGPENDING.