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@xxxxxxxxxx> 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@xxxxxxxx> > >>>>>>> > >>>>>>> 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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
