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Date: Mon, 6 Jun 2022 18:20:35 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Seth Forshee <sforshee@...italocean.com>,
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>,
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 6/6/22 16:13, Seth Forshee wrote:
> On Wed, May 04, 2022 at 01:08:40PM -0500, 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>
>
> Friendly reminder as it seems like this patch may have been forgotten.
Probably AB-BA maintainer deadlock. I have queued it now.
Paolo
> Thanks,
> Seth
>
>> ---
>> kernel/entry/kvm.c | 6 ------
>> 1 file changed, 6 deletions(-)
>>
>> diff --git a/kernel/entry/kvm.c b/kernel/entry/kvm.c
>> index 9d09f489b60e..2e0f75bcb7fd 100644
>> --- a/kernel/entry/kvm.c
>> +++ b/kernel/entry/kvm.c
>> @@ -9,12 +9,6 @@ static int xfer_to_guest_mode_work(struct kvm_vcpu *vcpu, unsigned long ti_work)
>> 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) {
>> kvm_handle_signal_exit(vcpu);
>> return -EINTR;
>> }
>> --
>> 2.32.0
>>
>
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