Re: [PATCH v2 01/12] KVM: s390: leave AIs in IPM of GISA during vcpu_pre_run()

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On Wed, 21 Nov 2018 09:30:34 +0100
Michael Mueller <mimu@xxxxxxxxxxxxx> wrote:

> On 20.11.18 12:33, Cornelia Huck wrote:
> > On Mon, 19 Nov 2018 18:25:25 +0100
> > Michael Mueller <mimu@xxxxxxxxxxxxx> wrote:
> >   
> >> Do not call __deliver_io() for adapter interruptions already
> >> pending in the IPM. That is a double effort. They will
> >> be processed as soon the vcpu control is given to SIE.
> >>
> >> Signed-off-by: Michael Mueller <mimu@xxxxxxxxxxxxx>
> >> ---
> >>   arch/s390/kvm/interrupt.c | 54 ++++++++++++++++++++++-------------------------
> >>   1 file changed, 25 insertions(+), 29 deletions(-)  
> > 
> > I think this patch does what it says on the tin, but I'm a bit lost as
> > to the why. (Might make more sense with the gib.)
> > 
> > Currently, we are trying to process any I/O interrupts, even if we'd
> > get them delivered via the gisa, when we're out of the SIE anyway.
> > IIRC, while this looks a bit like a belt-and-suspenders approach, it
> > also prevented performance problems when the vcpu did not go back into
> > the SIE immediately (it even may exit to userspace).
> > 
> > Also, if you're ignoring the I/O interrupts pending in the ipm, you may
> > end up delivering interrupts with a lower priority (higher isc) first.
> > I'm not sure that's what we want.
> > 
> > But maybe I'm just missing another bit of the code that makes this
> > safe. Can you elaborate a bit?  
> 
> 
> Function kvm_s390_deliver_pending_interrupts() is called in the 
> beginning of vcpu_pre_run() and we are about to enter the SIE anyway. 
> SIE will also grant the right ISC priority for adapter interruptions.
> 
> I basically want to avoid that a GISA that is part of the alert list
> will get its IPM bits cleared outside the GIB alert interruption 
> context. process_gib_alert_list()

Unfinished sentence?

Anyway, I think the patch description would benefit from adding a
sentence or two describing possible performance benefits and ease of
alert handling when the gib is introduced (?)



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