linux-kernel - Re: [patch V5 15/15] x86/kvm: Use generic xfer to guest work function

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Date:   Fri, 24 Jul 2020 21:08:34 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        linux-arch@...r.kernel.org, Will Deacon <will@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Mark Rutland <mark.rutland@....com>,
        Kees Cook <keescook@...omium.org>,
        Keno Fischer <keno@...iacomputing.com>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        Gabriel Krisman Bertazi <krisman@...labora.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [patch V5 15/15] x86/kvm: Use generic xfer to guest work function

Ingo Molnar <mingo@...nel.org> writes:
> * Thomas Gleixner <tglx@...utronix.de> wrote:
>>  		/*
>> -		 * Note, return 1 and not 0, vcpu_run() is responsible for
>> -		 * morphing the pending signal into the proper return code.
>> +		 * Note, return 1 and not 0, vcpu_run() will invoke
>> +		 * xfer_to_guest_mode() which will create a proper return
>> +		 * code.
>>  		 */
>> -		if (signal_pending(current))
>> +		if (__xfer_to_guest_mode_work_pending())
>>  			return 1;
>> -
>> -		if (need_resched())
>> -			schedule();
>>  	}
>
> AFAICS this chunk removes a conditional reschedule point from 
> handle_invalid_guest_state() and replaces it with 
> __xfer_to_guest_mode_work_pending().
>
> But __xfer_to_guest_mode_work_pending() doesn't do the cond-resched of 
> the full xfer_to_guest_mode_work() function - so we essentially lose a 
> cond_resched() here.
>
> Is this side effect intended, was the cond_resched() superfluous?

It makes the thing drop back to the outer loop for any pending work not
only for signals. That avoids having yet another thing to worry about.

Thanks,

        tglx