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. David -- 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
