Re: [PATCHv6] x86/xen: allow privcmd hypercalls to be preempted

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 02/19/2015 10:23 AM, David Vrabel wrote:
> Hypercalls submitted by user space tools via the privcmd driver can
> take a long time (potentially many 10s of seconds) if the hypercall
> has many sub-operations.
>
> A fully preemptible kernel may deschedule such as task in any upcall
> called from a hypercall continuation.
>
> However, in a kernel with voluntary or no preemption, hypercall
> continuations in Xen allow event handlers to be run but the task
> issuing the hypercall will not be descheduled until the hypercall is
> complete and the ioctl returns to user space.  These long running
> tasks may also trigger the kernel's soft lockup detection.
>
> Add xen_preemptible_hcall_begin() and xen_preemptible_hcall_end() to
> bracket hypercalls that may be preempted.  Use these in the privcmd
> driver.
>
> When returning from an upcall, call xen_maybe_preempt_hcall() which
> adds a schedule point if if the current task was within a preemptible
> hypercall.
>
> Since _cond_resched() can move the task to a different CPU, clear and
> set xen_in_preemptible_hcall around the call.
>
> Signed-off-by: David Vrabel <david.vrabel@citrix.com>

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>