Re: [PATCH] xen: privcmd: schedule() after private hypercall when non CONFIG_PREEMPT

[David Vrabel <david.vrabel@citrix.com>]

On 01/12/14 15:44, Luis R. Rodriguez wrote:
> On Mon, Dec 1, 2014 at 10:18 AM, David Vrabel <david.vrabel@citrix.com> wrote:
>> 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.
> 
> OK so that addresses what Xen will do when using continuation and
> hypercall preemption, my concern here was that using
> preempt_schedule_irq() on CONFIG_PREEMPT=n kernels in the middle of a
> hypercall on the return from an interrupt (e.g., the timer interrupt)
> would still let the kernel preempt to tasks other than those related
> to Xen.

Um.  Why would that be a problem?  We do want to switch to any task the
Linux scheduler thinks is best.

David