Re: [Xen-devel] [PATCH v3.1 03/15] xen/x86: allow calling {sh/hap}_set_allocation with the idle domain

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

On Tue, Nov 01, 2016 at 10:45:05AM +0000, Tim Deegan wrote:
> At 10:34 -0600 on 31 Oct (1477910088), Jan Beulich wrote:
> > >>> On 29.10.16 at 10:59, <roger.pau@xxxxxxxxxx> wrote:
> > > ... and using the "preempted" parameter. The solution relies on just 
> > > calling
> > > softirq_pending if the current domain is the idle domain. If such 
> > > preemption
> > > happens, the caller should then call process_pending_softirqs in order to
> > > drain the pending softirqs, and then call {sh/hap}_set_allocation again to
> > > continue with it's execution.
> > > 
> > > This allows us to call *_set_allocation() when building domain 0.
> > > 
> > > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > > Acked-by: George Dunlap <george.dunlap@xxxxxxxxxx>
> > > ---
> > > Cc: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
> > 
> > Cc: Tim
> > 
> > > Cc: Jan Beulich <jbeulich@xxxxxxxx>
> > > Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > > ---
> > > Changes since v2:
> > >  - Fix commit message.
> > > ---
> > >  xen/arch/x86/mm/hap/hap.c       | 4 +++-
> > >  xen/arch/x86/mm/shadow/common.c | 4 +++-
> > >  2 files changed, 6 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/xen/arch/x86/mm/hap/hap.c b/xen/arch/x86/mm/hap/hap.c
> > > index f099e94..0645521 100644
> > > --- a/xen/arch/x86/mm/hap/hap.c
> > > +++ b/xen/arch/x86/mm/hap/hap.c
> > > @@ -379,7 +379,9 @@ hap_set_allocation(struct domain *d, unsigned int 
> > > pages, 
> > > int *preempted)
> > >              break;
> > >  
> > >          /* Check to see if we need to yield and try again */
> > > -        if ( preempted && hypercall_preempt_check() )
> > > +        if ( preempted &&
> > > +             (is_idle_vcpu(current) ? 
> > > softirq_pending(smp_processor_id()) :
> > > +                                      hypercall_preempt_check()) )
> 
> This is a bit clunky, and is open-coded twice.  Can the existing checks in
> hypercall_preempt_check() be made safe to run on the idle vcpu?
> If not, please make a wrapper to call here that DTRT on idle and
> non-idle, e.g. something like:

Yes, hypercall_preempt_check can be made safe to run on the idle vcpu, just 
by guarding the call to local_events_need_delivery with a !is_idle_vcpu. 
I'm not really in favor of doing that, because then the name of the 
function is misleading, hypercall_preempt_check could then be used even on 
non-hypercall contexts.

>  /*
>   * For long-running operations that may be in hypercall context or on
>   * the idle vcpu (e.g. during dom0 construction), check if there is
>   * background work to be done that should interrupt this operation.
>   */
>  static inline bool general_preempt_check(void)
>  {
>      return unlikely(softirq_pending(smp_processor_id()) ||
>                      (!is_idle_vcpu(current) && 
> local_events_need_delivery()));
>  }
>
> If you're feeling keen, you could convert hypercall_preempt_check() to
> an inline function and comment it too. :)

IMHO this is better, and I don't mind changing hypercall_preempt_check to an 
inline function :).

Roger.

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