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

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Message-Id: <89E42BCC-47A8-458B-B06A-D6A20D20512C@amacapital.net>
Date:   Fri, 2 Aug 2019 15:39:22 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        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 Aug 2, 2019, at 3:22 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
>> 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.
> 
> 

If we add a real API for this, can we make it, or a very similar API, work for exit_to_usermode_loop() too?  Maybe:

bool usermode_work_pending();
bool guestmode_work_pending();

void do_usermode_work();
void do_guestmode_work();

The first two are called with IRQs off.  The latter two are called with IRQs on.