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Subject: Re: [PATCH] x86/xen: remove unneeded preempt_disable() from
 xen_irq_enable()
To:     Jan Beulich <jbeulich@suse.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, xen-devel@lists.xenproject.org,
        x86@kernel.org, linux-kernel@vger.kernel.org
References: <20210921070226.32021-1-jgross@suse.com>
 <99e6cf1d-ce6a-83e5-2e43-12f6c1a89f3f@suse.com>
 <599f4e20-a25d-6cd0-ced5-f2deaf997535@suse.com>
 <6305dfb1-15bc-ce4b-cbf7-d44ef5c0456d@suse.com>
From:   Juergen Gross <jgross@suse.com>
Message-ID: <ccd495a5-ba43-2541-f4d0-fb44f5516a55@suse.com>
Date:   Tue, 21 Sep 2021 10:24:52 +0200
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From: Juergen Gross <jgross@suse.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>,
 Stefano Stabellini <sstabellini@kernel.org>,
 Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
 Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
 xen-devel@lists.xenproject.org, x86@kernel.org, linux-kernel@vger.kernel.org
Message-ID: <ccd495a5-ba43-2541-f4d0-fb44f5516a55@suse.com>
Subject: Re: [PATCH] x86/xen: remove unneeded preempt_disable() from
 xen_irq_enable()
References: <20210921070226.32021-1-jgross@suse.com>
 <99e6cf1d-ce6a-83e5-2e43-12f6c1a89f3f@suse.com>
 <599f4e20-a25d-6cd0-ced5-f2deaf997535@suse.com>
 <6305dfb1-15bc-ce4b-cbf7-d44ef5c0456d@suse.com>
In-Reply-To: <6305dfb1-15bc-ce4b-cbf7-d44ef5c0456d@suse.com>

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On 21.09.21 10:11, Jan Beulich wrote:
> On 21.09.2021 09:58, Juergen Gross wrote:
>> On 21.09.21 09:53, Jan Beulich wrote:
>>> On 21.09.2021 09:02, Juergen Gross wrote:
>>>> --- a/arch/x86/xen/irq.c
>>>> +++ b/arch/x86/xen/irq.c
>>>> @@ -57,24 +57,20 @@ asmlinkage __visible void xen_irq_enable(void)
>>>>    {
>>>>    	struct vcpu_info *vcpu;
>>>>   =20
>>>> -	/*
>>>> -	 * We may be preempted as soon as vcpu->evtchn_upcall_mask is
>>>> -	 * cleared, so disable preemption to ensure we check for
>>>> -	 * events on the VCPU we are still running on.
>>>> -	 */
>>>> -	preempt_disable();
>>>> -
>>>>    	vcpu =3D this_cpu_read(xen_vcpu);
>>>>    	vcpu->evtchn_upcall_mask =3D 0;
>>>>   =20
>>>> -	/* Doesn't matter if we get preempted here, because any
>>>> -	   pending event will get dealt with anyway. */
>>>> +	/*
>>>> +	 * Now preemption could happen, but this is only possible if an ev=
ent
>>>> +	 * was handled, so missing an event due to preemption is not
>>>> +	 * possible at all.
>>>> +	 * The worst possible case is to be preempted and then check event=
s
>>>> +	 * pending on the old vcpu, but this is not problematic.
>>>> +	 */
>>>
>>> I agree this isn't problematic from a functional perspective, but ...=

>>>
>>>>    	barrier(); /* unmask then check (avoid races) */
>>>>    	if (unlikely(vcpu->evtchn_upcall_pending))
>>>>    		xen_force_evtchn_callback();
>>>
>>> ... is a stray call here cheaper than ...
>>>
>>>> -
>>>> -	preempt_enable();
>>>
>>> ... the preempt_{dis,en}able() pair?
>>
>> The question is if a stray call in case of preemption (very unlikely)
>> is cheaper than the preempt_{dis|en}able() pair on each IRQ enabling.
>>
>> I'm quite sure removing the preempt_*() calls will be a net benefit.
>=20
> Well, yes, I agree. It would have been nice if the description pointed
> out the fact that preemption kicking in precisely here is very unlikely=
=2E
> But perhaps that's considered rather obvious ... The issue I'm having
> is with the prior comments: They indicated that preemption happening
> before the "pending" check would be okay, _despite_ the
> preempt_{dis,en}able() pair. One could view this as an indication that
> this pair was put there for another reason (e.g. to avoid the stray
> calls). But it may of course also be that the comment simply was stale.=


The comment is older than the preempt_*() calls.

Those were added 8 years ago claiming they'd prevent lost events, but
at the same time at lease one other patch was added which really
prevented lost events, so adding the preempt_*() calls might just have
been a guess at that time.

> Reviewed-by: Jan Beulich <jbeulich@suse.com>

Thanks,


Juergen


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