linux-kernel - Re: [PATCH] xen: privcmd: schedule() after private hypercall when non CONFIG

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Message-ID: <547C86BF.2040705@citrix.com>
Date:	Mon, 1 Dec 2014 15:18:23 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	"Luis R. Rodriguez" <mcgrof@...e.com>
CC:	Juergen Gross <jgross@...e.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"Luis R. Rodriguez" <mcgrof@...not-panic.com>,
	<konrad.wilk@...cle.com>, <boris.ostrovsky@...cle.com>,
	<xen-devel@...ts.xenproject.org>, <linux-kernel@...r.kernel.org>,
	<x86@...nel.org>, <kvm@...r.kernel.org>,
	Davidlohr Bueso <dbueso@...e.de>,
	Joerg Roedel <jroedel@...e.de>, Borislav Petkov <bp@...e.de>,
	Jan Beulich <JBeulich@...e.com>, Olaf Hering <ohering@...e.de>
Subject: Re: [PATCH] xen: privcmd: schedule() after private hypercall when
 non CONFIG_PREEMPT

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@...e.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
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