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

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On 11/20/2018 12:33 PM, 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

In fact, doing the inject when in SIE is likely better performance-wise.
Right now we "inject" the floating interrupt and then we handle 
the requests. That can actually mean that it could take a while 
until the interrupt is actually noticed by the guest (when
in SIE). If you now have a 2nd CPU enabled this interrupt could
have been delivered to the guest much earlier but it is "stuck" in
the local CPU.


> 
> 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.

FWIW, LPAR has the same relaxation regarding priorities of subclasses.

> But maybe I'm just missing another bit of the code that makes this
> safe. Can you elaborate a bit?





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