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

["Luis R. Rodriguez" <mcgrof@suse.com>]

On Mon, Dec 01, 2014 at 03:54:24PM +0000, David Vrabel wrote:
> 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.

Its safe but -- it technically is doing kernel preemption, unless we want
to adjust the definition of CONFIG_PREEMPT=n to exclude hypercalls. This
was my original concern with the use of preempt_schedule_irq() to do this.
I am afraid of setting precedents without being clear or wider review and
acceptance.

  Luis