From mboxrd@z Thu Jan  1 00:00:00 1970
From: Juergen Gross <jgross@suse.com>
Subject: Re: [Xen-devel] [PATCH] xen: privcmd: schedule() after private hypercall
 when non CONFIG_PREEMPT
Date: Wed, 03 Dec 2014 05:37:51 +0100
Message-ID: <547E939F.40106@suse.com>
References: <20141127183616.GV25677@wotan.suse.de> <547C4CEF.1010603@citrix.com> <20141201150546.GC25677@wotan.suse.de> <547C86BF.2040705@citrix.com> <CAB=NE6WPvCJnAfdbZqcw5pJtZrfxo0zQu6J2gpXp69G-UTtiUg@mail.gmail.com> <547C8F30.1010306@citrix.com> <20141201161905.GH25677@wotan.suse.de> <547CB07C.1050507@citrix.com> <20141201223611.GM25677@wotan.suse.de> <547D9E56.50708@citrix.com> <20141203022833.GS25677@wotan.suse.de>
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Cc: Joerg Roedel <jroedel@suse.de>, kvm@vger.kernel.org,
	Davidlohr Bueso <dbueso@suse.de>,
	Peter Zijlstra <peterz@infradead.org>, x86@kernel.org,
	Oleg Nesterov <oleg@redhat.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Jan Beulich <JBeulich@suse.com>,
	xen-devel@lists.xenproject.org, boris.ostrovsky@oracle.com,
	Borislav Petkov <bp@suse.de>, Olaf Hering <ohering@suse.de>,
	Ingo Molnar <mingo@kernel.org>
To: "Luis R. Rodriguez" <mcgrof@suse.com>,
	David Vrabel <david.vrabel@citrix.com>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <20141203022833.GS25677@wotan.suse.de>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: kvm.vger.kernel.org

On 12/03/2014 03:28 AM, Luis R. Rodriguez wrote:
> On Tue, Dec 02, 2014 at 11:11:18AM +0000, David Vrabel wrote:
>> On 01/12/14 22:36, Luis R. Rodriguez wrote:
>>>
>>> Then I do agree its a fair analogy (and find this obviously odd that how
>>> widespread cond_resched() is), we just don't have an equivalent for IRQ
>>> context, why not avoid the special check then and use this all the time in the
>>> middle of a hypercall on the return from an interrupt (e.g., the timer
>>> interrupt)?
>>
>> http://lists.xen.org/archives/html/xen-devel/2014-02/msg01101.html
>
> OK thanks! That explains why we need some asm code but in that submission you
> still also had used is_preemptible_hypercall(regs) and in the new
> implementation you use a CPU variable xen_in_preemptible_hcall prior to calling
> preempt_schedule_irq(). I believe you added the CPU variable because
> preempt_schedule_irq() will preempt first without any checks if it should, I'm
> asking why not do something like cond_resched_irq() where we check with
> should_resched() prior to preempting and that way we can avoid having to use
> the CPU variable?

Because that could preempt at any asynchronous interrupt making the
no-preempt kernel fully preemptive. How would you know you are just
doing a critical hypercall which should be preempted?

Juergen