From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Vrabel <david.vrabel@citrix.com>
Subject: Re: [PATCH] xen: privcmd: schedule() after private hypercall when
 non CONFIG_PREEMPT
Date: Mon, 1 Dec 2014 15:18:23 +0000
Message-ID: <547C86BF.2040705@citrix.com>
References: <1417040805-15857-1-git-send-email-mcgrof@do-not-panic.com> <5476C66F.5040308@suse.com> <20141127183616.GV25677@wotan.suse.de> <547C4CEF.1010603@citrix.com> <20141201150546.GC25677@wotan.suse.de>
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Cc: Juergen Gross <jgross@suse.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>,
	Oleg Nesterov <oleg@redhat.com>,
	"Luis R. Rodriguez" <mcgrof@do-not-panic.com>,
	<konrad.wilk@oracle.com>, <boris.ostrovsky@oracle.com>,
	<xen-devel@lists.xenproject.org>, <linux-kernel@vger.kernel.org>,
	<x86@kernel.org>, <kvm@vger.kernel.org>,
	Davidlohr Bueso <dbueso@suse.de>,
	Joerg Roedel <jroedel@suse.de>, Borislav Petkov <bp@suse.de>,
	Jan Beulich <JBeulich@suse.com>, Olaf Hering <ohering@suse.de>
To: "Luis R. Rodriguez" <mcgrof@suse.com>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <20141201150546.GC25677@wotan.suse.de>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: kvm.vger.kernel.org

On 01/12/14 15:05, Luis R. Rodriguez wrote:
> On Mon, Dec 01, 2014 at 11:11:43AM +0000, David Vrabel wrote:
>> On 27/11/14 18:36, Luis R. Rodriguez wrote:
>>> On Thu, Nov 27, 2014 at 07:36:31AM +0100, Juergen Gross wrote:
>>>> On 11/26/2014 11:26 PM, Luis R. Rodriguez wrote:
>>>>> From: "Luis R. Rodriguez" <mcgrof@suse.com>
>>>>>
>>>>> Some folks had reported that some xen hypercalls take a long time
>>>>> to complete when issued from the userspace private ioctl mechanism,
>>>>> this can happen for instance with some hypercalls that have many
>>>>> sub-operations, this can happen for instance on hypercalls that use
>> [...]
>>>>> --- a/drivers/xen/privcmd.c
>>>>> +++ b/drivers/xen/privcmd.c
>>>>> @@ -60,6 +60,9 @@ static long privcmd_ioctl_hypercall(void __user *udata)
>>>>>   			   hypercall.arg[0], hypercall.arg[1],
>>>>>   			   hypercall.arg[2], hypercall.arg[3],
>>>>>   			   hypercall.arg[4]);
>>>>> +#ifndef CONFIG_PREEMPT
>>>>> +	schedule();
>>>>> +#endif
>>
>> As Juergen points out, this does nothing.  You need to schedule while in
>> the middle of the hypercall.
>>
>> Remember that Xen's hypercall preemption only preempts the hypercall to
>> run interrupts in the guest.
> 
> How is it ensured that when the kernel preempts on this code path on
> CONFIG_PREEMPT=n kernel that only interrupts in the guest are run?

Sorry, I really didn't describe this very well.

If a hypercall needs a continuation, Xen returns to the guest with the
IP set to the hypercall instruction, and on the way back to the guest
Xen may schedule a different VCPU or it will do any upcalls (as per normal).

The guest is free to return from the upcall to the original task
(continuing the hypercall) or to a different one.

David