linux-kernel - Re: [patch 2/5] x86/kvm: Handle task

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Date:   Sat, 3 Aug 2019 00:22:54 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Paolo Bonzini <pbonzini@...hat.com>
cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Oleg Nesterov <oleg@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Julia Cartwright <julia@...com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Frederic Weisbecker <fweisbec@...il.com>, kvm@...r.kernel.org,
        Radim Krcmar <rkrcmar@...hat.com>,
        John Stultz <john.stultz@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: Re: [patch 2/5] x86/kvm: Handle task_work on VMENTER/EXIT

On Fri, 2 Aug 2019, Paolo Bonzini wrote:
> On 01/08/19 23:47, Thomas Gleixner wrote:
> > Right you are about cond_resched() being called, but for SRCU this does not
> > matter unless there is some way to do a synchronize operation on that SRCU
> > entity. It might have some other performance side effect though.
> 
> I would use srcu_read_unlock/lock around the call.
> 
> However, I'm wondering if the API can be improved because basically we
> have six functions for three checks of TIF flags.  Does it make sense to
> have something like task_has_request_flags and task_do_requests (names
> are horrible I know) that can be used like
> 
> 	if (task_has_request_flags()) {
> 		int err;
> 		...srcu_read_unlock...
> 		// return -EINTR if signal_pending
> 		err = task_do_requests();
> 		if (err < 0)
> 			goto exit_no_srcu_read_unlock;
> 		...srcu_read_lock...
> 	}
> 
> taking care of all three cases with a single hook?  This is important
> especially because all these checks are done by all KVM architectures in
> slightly different ways, and a unified API would be a good reason to
> make all architectures look the same.
> 
> (Of course I could also define this unified API in virt/kvm/kvm_main.c,
> so this is not blocking the series in any way!).

You're not holding up something. Having a common function for this is
definitely the right approach.

As this is virt specific because it only checks for non arch specific bits
(TIF_NOTIFY_RESUME should be available for all KVM archs) and the TIF bits
are a subset of the available TIF bits because all others do not make any
sense there, this really should be a common function for KVM so that all
other archs which obviously lack a TIF_NOTIFY_RESUME check, can be fixed up
and consolidated. If we add another TIF check later then we only have to do
it in one place.

Thanks,

	tglx